apollo_federation/connectors/runtime/

responses.rs

use apollo_compiler::ExecutableDocument;
use apollo_compiler::Name;
use apollo_compiler::collections::HashMap;
use apollo_compiler::collections::IndexMap;
use apollo_compiler::collections::IndexSet;
use apollo_compiler::executable::Selection;
use apollo_compiler::executable::SelectionSet;
use encoding_rs::Encoding;
use encoding_rs::UTF_8;
use http::HeaderMap;
use http::HeaderValue;
use http::header::CONTENT_LENGTH;
use http::header::CONTENT_TYPE;
use http::response::Parts;
use itertools::Itertools;
use mime::Mime;
use serde_json_bytes::ByteString;
use serde_json_bytes::Map;
use serde_json_bytes::Value;

use crate::connectors::Connector;
use crate::connectors::JSONSelection;
use crate::connectors::ProblemLocation;
use crate::connectors::runtime::errors::RuntimeError;
use crate::connectors::runtime::inputs::ContextReader;
use crate::connectors::runtime::key::ResponseKey;
use crate::connectors::runtime::mapping::Problem;
use crate::connectors::runtime::mapping::aggregate_apply_to_errors;
use crate::connectors::runtime::responses::DeserializeError::ContentDecoding;

const ENTITIES: &str = "_entities";
const TYPENAME: &str = "__typename";

#[derive(Debug, thiserror::Error)]
pub enum HandleResponseError {
    #[error("Merge error: {0}")]
    MergeError(String),
}

/// Converts a response body into a json Value based on the Content-Type header.
pub fn deserialize_response(body: &[u8], headers: &HeaderMap) -> Result<Value, DeserializeError> {
    // If the body is empty, there's nothing to parse. We check body.is_empty()
    // directly because spec-compliant HTTP 204 responses must not include a
    // Content-Length header — so we can't rely on that header alone to detect
    // empty bodies. The Content-Length: 0 check is kept for non-compliant
    // servers that do send it, but body.is_empty() covers both cases.
    if body.is_empty()
        || headers
            .get(CONTENT_LENGTH)
            .and_then(|len| len.to_str().ok())
            .and_then(|s| s.parse::<usize>().ok())
            .is_some_and(|content_length| content_length == 0)
    {
        return Ok(Value::Null);
    }

    let content_type = headers
        .get(CONTENT_TYPE)
        .and_then(|h| h.to_str().ok()?.parse::<Mime>().ok());

    if content_type.is_none()
        || content_type
            .as_ref()
            .is_some_and(|ct| ct.subtype() == mime::JSON || ct.suffix() == Some(mime::JSON))
    {
        // Treat any JSON-y like content types as JSON
        // Also, because the HTTP spec says we should effectively "guess" the content type if there is no content type (None), we're going to guess it is JSON if the server has not specified one
        serde_json::from_slice::<Value>(body).map_err(DeserializeError::SerdeJson)
    } else if content_type
        .as_ref()
        .is_some_and(|ct| ct.type_() == mime::TEXT && ct.subtype() == mime::PLAIN)
    {
        // Plain text we can't parse as JSON so we'll instead return it as a JSON string
        // Before we can do that, we need to figure out the charset and attempt to decode the string
        let encoding = content_type
            .as_ref()
            .and_then(|ct| Encoding::for_label(ct.get_param("charset")?.as_str().as_bytes()))
            .unwrap_or(UTF_8);
        let (decoded_body, _, had_errors) = encoding.decode(body);

        if had_errors {
            return Err(ContentDecoding(encoding.name()));
        }

        Ok(Value::String(decoded_body.into_owned().into()))
    } else {
        // For any other content types, all we can do is treat it as a JSON null cause we don't know what it is
        Ok(Value::Null)
    }
}

#[derive(Debug, thiserror::Error)]
pub enum DeserializeError {
    #[error("Could not parse JSON: {0}")]
    SerdeJson(#[source] serde_json::Error),
    #[error("Could not decode data with content encoding {0}")]
    ContentDecoding(&'static str),
}

pub fn handle_raw_response(
    data: &Value,
    parts: &Parts,
    key: ResponseKey,
    connector: &Connector,
    context: impl ContextReader,
    client_headers: &HeaderMap<HeaderValue>,
) -> MappedResponse {
    let inputs = key
        .inputs()
        .clone()
        .merger(&connector.response_variable_keys)
        .config(connector.config.as_ref())
        .context(context)
        .status(parts.status.as_u16())
        .request(&connector.response_headers, client_headers)
        .response(&connector.response_headers, Some(parts))
        .merge();
    let warnings = Vec::new();
    let (success, warnings) = is_success(connector, data, parts, &inputs, warnings);
    if success {
        map_response(data, key, inputs, warnings)
    } else {
        map_error(connector, data, parts, key, inputs, warnings)
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct GraphQLDataMapper<'a> {
    doc: &'a ExecutableDocument,
    subtypes_map: &'a IndexMap<String, IndexSet<String>>,
}

impl<'a> GraphQLDataMapper<'a> {
    fn new(
        doc: &'a ExecutableDocument,
        subtypes_map: &'a IndexMap<String, IndexSet<String>>,
    ) -> Self {
        Self { doc, subtypes_map }
    }

    fn fragment_matches(&self, data: &Value, fragment_type_condition: &Name) -> bool {
        if let Some(data_typename) = data.get("__typename") {
            match data_typename {
                Value::String(typename) => {
                    self.supertype_has_subtype(fragment_type_condition.as_str(), typename.as_str())
                }
                _ => false,
            }
        } else {
            true
        }
    }

    fn supertype_has_subtype(&self, supertype: &str, subtype: &str) -> bool {
        if supertype == subtype {
            true
        } else if let Some(subtypes) = self.subtypes_map.get(supertype) {
            subtypes
                .iter()
                .any(|s| self.supertype_has_subtype(s, subtype))
        } else {
            false
        }
    }

    fn map_data(&self, data: &Value, selection_set: &SelectionSet) -> Value {
        if selection_set.selections.is_empty() {
            return data.clone();
        }

        match data {
            Value::Object(map) => {
                let mut new_map = Map::new();

                for field in selection_set.selections.iter() {
                    match field {
                        Selection::Field(field) => {
                            if let Some(field_value) = map.get(field.name.as_str()) {
                                let output_field_name = field.alias.as_ref().unwrap_or(&field.name);
                                new_map.insert(
                                    output_field_name.to_string(),
                                    self.map_data(field_value, &field.selection_set),
                                );
                            } else if field.name == TYPENAME {
                                // __typename is an intrinsic field that always
                                // resolves to the concrete type name, even when
                                // the connector response doesn't include it
                                // (e.g., mappingOnly connectors returning `{}`).
                                let output_field_name = field.alias.as_ref().unwrap_or(&field.name);
                                new_map.insert(
                                    output_field_name.to_string(),
                                    Value::String(selection_set.ty.to_string().into()),
                                );
                            }
                        }

                        Selection::FragmentSpread(spread) => {
                            if let Some(fragment) =
                                self.doc.fragments.get(spread.fragment_name.as_str())
                                && self.fragment_matches(data, fragment.type_condition())
                            {
                                let mapped = self.map_data(data, &fragment.selection_set);
                                if let Some(fragment_map) = mapped.as_object() {
                                    new_map.extend(fragment_map.clone());
                                }
                            }
                        }

                        Selection::InlineFragment(fragment) => {
                            if let Some(type_condition) = &fragment.type_condition
                                && !self.fragment_matches(data, type_condition)
                            {
                                continue;
                            }
                            let mapped = self.map_data(data, &fragment.selection_set);
                            if let Some(fragment_map) = mapped.as_object() {
                                new_map.extend(fragment_map.clone());
                            }
                        }
                    }
                }

                Value::Object(new_map)
            }

            Value::Array(items) => Value::Array(
                items
                    .iter()
                    .map(|item| self.map_data(item, selection_set))
                    .collect(),
            ),

            primitive => primitive.clone(),
        }
    }
}

// If the user has set a custom success condition selector, resolve that expression,
// otherwise default to checking status code is 2XX
fn is_success(
    connector: &Connector,
    data: &Value,
    parts: &Parts,
    inputs: &IndexMap<String, Value>,
    mut warnings: Vec<Problem>,
) -> (bool, Vec<Problem>) {
    let Some(is_success_selection) = &connector.error_settings.connect_is_success else {
        return (parts.status.is_success(), warnings);
    };
    let (res, apply_to_errors) = is_success_selection.apply_with_vars(data, inputs);
    warnings.extend(aggregate_apply_to_errors(
        apply_to_errors,
        ProblemLocation::IsSuccess,
    ));

    (
        res.as_ref().and_then(Value::as_bool).unwrap_or_default(),
        warnings,
    )
}

/// Returns a response for a mapping-only connector by applying the selection against `{}`.
///
/// Used when `http` is omitted from a `@connect` directive, skipping the HTTP transport.
pub fn handle_mapping_only_response(
    key: ResponseKey,
    connector: &Connector,
    context: impl ContextReader,
    client_headers: &HeaderMap<HeaderValue>,
) -> MappedResponse {
    let data = Value::Object(Map::new());
    let inputs = key
        .inputs()
        .clone()
        .merger(&connector.response_variable_keys)
        .config(connector.config.as_ref())
        .context(context)
        .request(&connector.response_headers, client_headers)
        .merge();
    map_response(&data, key, inputs, Vec::new())
}

/// Returns a response with data transformed by the selection mapping.
pub(super) fn map_response(
    data: &Value,
    key: ResponseKey,
    inputs: IndexMap<String, Value>,
    mut warnings: Vec<Problem>,
) -> MappedResponse {
    let (res, apply_to_errors) = key.selection().apply_with_vars(data, &inputs);
    warnings.extend(aggregate_apply_to_errors(
        apply_to_errors,
        ProblemLocation::Selection,
    ));
    MappedResponse::Data {
        key,
        data: res.unwrap_or_else(|| Value::Null),
        problems: warnings,
    }
}

/// Returns a `MappedResponse` with a GraphQL error.
pub(super) fn map_error(
    connector: &Connector,
    data: &Value,
    parts: &Parts,
    key: ResponseKey,
    inputs: IndexMap<String, Value>,
    mut warnings: Vec<Problem>,
) -> MappedResponse {
    // Do we have an error message mapping set for this connector?
    let message = if let Some(message_selection) = &connector.error_settings.message {
        let (res, apply_to_errors) = message_selection.apply_with_vars(data, &inputs);
        warnings.extend(aggregate_apply_to_errors(
            apply_to_errors,
            ProblemLocation::ErrorsMessage,
        ));
        res.as_ref()
            .and_then(Value::as_str)
            .unwrap_or_default()
            .to_string()
    } else {
        "Request failed".to_string()
    };

    // Now we can create the error object using either the default message or the message calculated by the JSONSelection
    let mut error = RuntimeError::new(message, &key);
    error.subgraph_name = Some(connector.id.subgraph_name.clone());
    error.coordinate = Some(connector.id.coordinate());

    // First, we will apply defaults... these may get overwritten below by user configured extensions
    error = error.extension(
        "http",
        Value::Object(Map::from_iter([(
            "status".into(),
            Value::Number(parts.status.as_u16().into()),
        )])),
    );

    // If we have error extensions mapping set for this connector, we will need to grab the code + the remaining extensions and map them to the error object
    // We'll merge by applying the source and then the connect. Keep in mind that these will override defaults if the key names are the same.
    // Note: that we set the extension code in this if/else but don't actually set it on the error until after the if/else. This is because the compiler
    // can't make sense of it in the if/else due to how the builder is constructed.
    let mut extension_code = "CONNECTOR_FETCH".to_string();
    if let Some(extensions_selection) = &connector.error_settings.source_extensions {
        let (res, apply_to_errors) = extensions_selection.apply_with_vars(data, &inputs);
        warnings.extend(aggregate_apply_to_errors(
            apply_to_errors,
            ProblemLocation::SourceErrorsExtensions,
        ));

        // TODO: Currently this "fails silently". In the future, we probably add a warning to the debugger info.
        let extensions = res
            .and_then(|e| match e {
                Value::Object(map) => Some(map),
                _ => None,
            })
            .unwrap_or_default();

        if let Some(code) = extensions.get("code") {
            extension_code = code.as_str().unwrap_or_default().to_string();
        }

        for (key, value) in extensions {
            error = error.extension(key, value);
        }
    }

    if let Some(extensions_selection) = &connector.error_settings.connect_extensions {
        let (res, apply_to_errors) = extensions_selection.apply_with_vars(data, &inputs);
        warnings.extend(aggregate_apply_to_errors(
            apply_to_errors,
            ProblemLocation::ConnectErrorsExtensions,
        ));

        // TODO: Currently this "fails silently". In the future, we probably add a warning to the debugger info.
        let extensions = res
            .and_then(|e| match e {
                Value::Object(map) => Some(map),
                _ => None,
            })
            .unwrap_or_default();

        if let Some(code) = extensions.get("code") {
            extension_code = code.as_str().unwrap_or_default().to_string();
        }

        for (key, value) in extensions {
            error = error.extension(key, value);
        }
    }

    error = error.with_code(extension_code);

    MappedResponse::Error {
        error,
        key,
        problems: warnings,
    }
}
// --- MAPPED RESPONSE ---------------------------------------------------------
#[derive(Debug)]
pub enum MappedResponse {
    /// This is equivalent to RawResponse::Error, but it also represents errors
    /// when the request is semantically unsuccessful (e.g. 404, 500).
    Error {
        error: RuntimeError,
        key: ResponseKey,
        problems: Vec<Problem>,
    },
    /// The response data after applying the selection mapping.
    Data {
        data: Value,
        key: ResponseKey,
        problems: Vec<Problem>,
    },
}

impl MappedResponse {
    /// Adds the response data to the `data` map or the error to the `errors`
    /// array. How data is added depends on the `ResponseKey`: it's either a
    /// property directly on the map, or stored in the `_entities` array.
    pub fn add_to_data(
        self,
        data: &mut Map<ByteString, Value>,
        errors: &mut Vec<RuntimeError>,
        count: usize,
    ) -> Result<(), HandleResponseError> {
        match self {
            Self::Error { error, key, .. } => {
                match key {
                    // add a null to the "_entities" array at the right index
                    ResponseKey::Entity { index, .. } | ResponseKey::EntityField { index, .. } => {
                        let entities = data
                            .entry(ENTITIES)
                            .or_insert(Value::Array(Vec::with_capacity(count)));
                        entities
                            .as_array_mut()
                            .ok_or_else(|| {
                                HandleResponseError::MergeError("_entities is not an array".into())
                            })?
                            .insert(index, Value::Null);
                    }
                    _ => {}
                };
                errors.push(error);
            }
            Self::Data {
                data: value, key, ..
            } => match key {
                ResponseKey::RootField { ref name, .. } => {
                    data.insert(name.clone(), value);
                }
                ResponseKey::Entity { index, .. } => {
                    let entities = data
                        .entry(ENTITIES)
                        .or_insert(Value::Array(Vec::with_capacity(count)));
                    entities
                        .as_array_mut()
                        .ok_or_else(|| {
                            HandleResponseError::MergeError("_entities is not an array".into())
                        })?
                        .insert(index, value);
                }
                ResponseKey::EntityField {
                    index,
                    ref field_name,
                    ref typename,
                    ..
                } => {
                    let entities = data
                        .entry(ENTITIES)
                        .or_insert(Value::Array(Vec::with_capacity(count)))
                        .as_array_mut()
                        .ok_or_else(|| {
                            HandleResponseError::MergeError("_entities is not an array".into())
                        })?;

                    match entities.get_mut(index) {
                        Some(Value::Object(entity)) => {
                            entity.insert(field_name.clone(), value);
                        }
                        _ => {
                            let mut entity = Map::new();
                            if let Some(typename) = typename {
                                entity.insert(TYPENAME, Value::String(typename.as_str().into()));
                            }
                            entity.insert(field_name.clone(), value);
                            entities.insert(index, Value::Object(entity));
                        }
                    };
                }
                ResponseKey::BatchEntity {
                    selection,
                    keys,
                    inputs,
                } => {
                    let Value::Array(values) = value else {
                        return Err(HandleResponseError::MergeError(
                            "Response for a batch request does not map to an array".into(),
                        ));
                    };

                    let spec = selection.spec();
                    let key_selection = JSONSelection::parse_with_spec(
                        &keys.serialize().no_indent().to_string(),
                        spec,
                    )
                    .map_err(|e| HandleResponseError::MergeError(e.to_string()))?;

                    // Convert representations into keys for use in the map
                    let key_values = inputs.batch.iter().map(|v| {
                        key_selection
                            .apply_to(&Value::Object(v.clone()))
                            .0
                            .unwrap_or(Value::Null)
                    });

                    // Create a map of keys to entities
                    let mut map = values
                        .into_iter()
                        .filter_map(|v| key_selection.apply_to(&v).0.map(|key| (key, v)))
                        .collect::<HashMap<_, _>>();

                    // Make a list of entities that matches the representations list
                    let new_entities = key_values
                        .map(|key| map.remove(&key).unwrap_or(Value::Null))
                        .collect_vec();

                    // Because we may have multiple batch entities requests, we should add to ENTITIES as the requests come in so it is additive
                    let entities = data
                        .entry(ENTITIES)
                        .or_insert(Value::Array(Vec::with_capacity(count)));

                    entities
                        .as_array_mut()
                        .ok_or_else(|| {
                            HandleResponseError::MergeError("_entities is not an array".into())
                        })?
                        .extend(new_entities);
                }
            },
        }

        Ok(())
    }

    pub fn problems(&self) -> &[Problem] {
        match self {
            Self::Error { problems, .. } | Self::Data { problems, .. } => problems,
        }
    }

    /// Applies the given GraphQL operation (note: must be a single operation!)
    /// to the [`MappedResponse`] to produce a new [`MappedResponse`] with
    /// GraphQL transforms like alias renaming applied.
    ///
    /// The `operation_option` parameter is an [`Option<&ExecutableDocument>`]
    /// to simplify cases where you might not have an [`ExecutableDocument`]
    /// available (hence `None`). When `operation_option.is_none()`, note that
    /// `subtypes` is ignored.
    ///
    /// The `subtypes` parameter is necessary for handling abstract fragment
    /// type conditions, since that information is not preserved in
    /// [`ExecutableDocument`].
    pub fn apply_operation(
        self, // NOTE: Takes ownership of self!
        operation_option: Option<&ExecutableDocument>,
        subtypes: &IndexMap<String, IndexSet<String>>,
    ) -> Self {
        match (self, operation_option) {
            (
                Self::Data {
                    data,
                    key,
                    problems,
                },
                Some(operation),
            ) => {
                let single_op = operation
                    .operations
                    .anonymous
                    .as_ref()
                    .or_else(|| operation.operations.named.values().next());

                let data = if let Some(op) = single_op {
                    let mut new_sub = SelectionSet::new(op.selection_set.ty.clone());

                    match &key {
                        ResponseKey::RootField { name, .. } => {
                            for field in op.selection_set.selections.iter() {
                                if let Selection::Field(field) = field
                                    && field.alias.as_deref().unwrap_or(field.name.as_str())
                                        == name.as_str()
                                {
                                    // Use the field's selection set type so that
                                    // __typename resolves to the return type (e.g.
                                    // "UserMutations") rather than the root operation
                                    // type (e.g. "Mutation").
                                    new_sub.ty = field.selection_set.ty.clone();
                                    new_sub
                                        .selections
                                        .extend(field.selection_set.selections.iter().cloned());
                                }
                            }
                        }

                        ResponseKey::EntityField { field_name, .. } => {
                            let field_str = field_name.as_str();

                            for selection in op.selection_set.selections.iter() {
                                if let Selection::Field(field) = selection
                                    && field.name.as_str() == "_entities"
                                {
                                    for ent_sel in field.selection_set.selections.iter() {
                                        // Selection::InlineFragment is what we
                                        // actually expect, but we could handle
                                        // ::Field and ::FragmentSpread too if
                                        // necessary.
                                        match ent_sel {
                                            Selection::InlineFragment(frag) => {
                                                for field_sel in
                                                    frag.selection_set.selections.iter()
                                                {
                                                    if let Selection::Field(field) = field_sel
                                                        && field.name.as_str() == field_str
                                                    {
                                                        new_sub.selections.extend(
                                                            field
                                                                .selection_set
                                                                .selections
                                                                .iter()
                                                                .cloned(),
                                                        );
                                                    }
                                                }
                                            }

                                            Selection::Field(field) => {
                                                if field.name.as_str() == field_str {
                                                    new_sub.selections.extend(
                                                        field
                                                            .selection_set
                                                            .selections
                                                            .iter()
                                                            .cloned(),
                                                    );
                                                }
                                            }

                                            Selection::FragmentSpread(spread) => {
                                                if let Some(fragment) = operation
                                                    .fragments
                                                    .get(spread.fragment_name.as_str())
                                                {
                                                    for field_sel in
                                                        fragment.selection_set.selections.iter()
                                                    {
                                                        if let Selection::Field(field) = field_sel
                                                            && field.name.as_str() == field_str
                                                        {
                                                            new_sub.selections.extend(
                                                                field
                                                                    .selection_set
                                                                    .selections
                                                                    .iter()
                                                                    .cloned(),
                                                            );
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    }
                                }
                            }
                        }

                        ResponseKey::Entity { .. } => {
                            for selection in op.selection_set.selections.iter() {
                                if let Selection::Field(field) = selection
                                    && field.name.as_str() == "_entities"
                                {
                                    new_sub
                                        .selections
                                        .extend(field.selection_set.selections.iter().cloned());
                                }
                            }
                        }

                        ResponseKey::BatchEntity { keys, .. } => {
                            new_sub
                                .selections
                                .extend(keys.selection_set.selections.iter().cloned());

                            for selection in op.selection_set.selections.iter() {
                                if let Selection::Field(field) = selection
                                    && field.name.as_str() == "_entities"
                                {
                                    new_sub
                                        .selections
                                        .extend(field.selection_set.selections.iter().cloned());
                                }
                            }
                        }
                    };

                    GraphQLDataMapper::new(operation, subtypes).map_data(&data, &new_sub)
                } else {
                    data
                };

                Self::Data {
                    data,
                    key,
                    problems,
                }
            }

            // We do not transform errors using the operation.
            (
                MappedResponse::Error {
                    error,
                    key,
                    problems,
                },
                Some(_),
            ) => MappedResponse::Error {
                error,
                key,
                problems,
            },

            // When operation_option.is_none(), return self unmodified.
            (mapped, None) => mapped,
        }
    }
}

#[cfg(test)]
mod tests {
    use std::sync::Arc;

    use apollo_compiler::ExecutableDocument;
    use apollo_compiler::Schema;
    use http::HeaderMap;
    use http::HeaderValue;
    use serde_json_bytes::Value;
    use serde_json_bytes::json;

    use super::MappedResponse;
    use super::deserialize_response;
    use crate::connectors::JSONSelection;
    use crate::connectors::runtime::inputs::RequestInputs;
    use crate::connectors::runtime::key::ResponseKey;

    fn headers_with(pairs: &[(&str, &str)]) -> HeaderMap {
        let mut map = HeaderMap::new();
        for (k, v) in pairs {
            map.insert(
                http::header::HeaderName::from_bytes(k.as_bytes()).unwrap(),
                HeaderValue::from_str(v).unwrap(),
            );
        }
        map
    }

    #[test]
    fn empty_body_no_content_length_returns_null() {
        // Spec-compliant 204: no Content-Length header, no body.
        let headers = HeaderMap::new();
        let result = deserialize_response(b"", &headers).unwrap();
        assert_eq!(result, Value::Null);
    }

    #[test]
    fn empty_body_with_content_length_zero_returns_null() {
        // Non-compliant server that sends Content-Length: 0 on a 204.
        let headers = headers_with(&[("content-length", "0")]);
        let result = deserialize_response(b"", &headers).unwrap();
        assert_eq!(result, Value::Null);
    }

    #[test]
    fn test_apply_operation_with_root_and_field_aliases() {
        let schema = Schema::parse_and_validate(
            r#"
            type Query {
                search_items(query: String): SearchResponse
            }
            type SearchResponse {
                results: [Item!]!
                metadata: Metadata!
            }
            type Item {
                id: ID!
                title: String!
                viewUri: String!
            }
            type Metadata {
                total: Int!
            }
            "#,
            "schema.graphql",
        )
        .unwrap();

        let query = r#"
            {
                items:search_items(query: "test") {
                    results {
                        id
                        title
                        link:viewUri
                    }
                    metadata {
                        total
                    }
                }
            }
            "#;

        let operation =
            ExecutableDocument::parse_and_validate(&schema, query, "op.graphql").unwrap();

        let mapped_data = json!({
            "results": [
                { "id": "1", "title": "First", "viewUri": "https://example.com/1" },
                { "id": "2", "title": "Second", "viewUri": "https://example.com/2" }
            ],
            "metadata": { "total": 2 }
        });

        let response = MappedResponse::Data {
            key: ResponseKey::RootField {
                name: "items".to_string(),
                inputs: RequestInputs::default(),
                selection: Arc::new(JSONSelection::parse("$").unwrap()),
            },
            data: mapped_data,
            problems: vec![],
        };

        let result = response.apply_operation(Some(&*operation), &Default::default());

        let MappedResponse::Data { data, .. } = result else {
            panic!("expected Data variant");
        };

        let items = data["results"].as_array().expect("results should be array");
        assert_eq!(items.len(), 2);

        // `link` (alias for viewUri) must be present; `viewUri` must not appear under the alias name.
        assert_eq!(
            items[0]["link"].as_str(),
            Some("https://example.com/1"),
            "field alias 'link' should resolve to viewUri value"
        );
        assert_eq!(
            items[1]["link"].as_str(),
            Some("https://example.com/2"),
            "field alias 'link' should resolve to viewUri value"
        );
        assert!(
            items[0].get("viewUri").is_none(),
            "original field name should not appear in output when aliased"
        );
    }
}