database_mcp_postgres/tools/
get_table_schema.rs

1//! MCP tool: `get_table_schema`.
2
3use std::borrow::Cow;
4use std::collections::HashMap;
5
6use database_mcp_server::AppError;
7use database_mcp_server::types::{GetTableSchemaRequest, TableSchemaResponse};
8use database_mcp_sql::Connection as _;
9use database_mcp_sql::identifier::validate_identifier;
10use rmcp::handler::server::router::tool::{AsyncTool, ToolBase};
11use rmcp::model::{ErrorData, ToolAnnotations};
12use serde_json::{Value, json};
13
14use crate::PostgresHandler;
15
16/// Marker type for the `get_table_schema` MCP tool.
17pub(crate) struct GetTableSchemaTool;
18
19impl GetTableSchemaTool {
20    const NAME: &'static str = "get_table_schema";
21    const TITLE: &'static str = "Get Table Schema";
22    const DESCRIPTION: &'static str = r#"Get column definitions and foreign key relationships for a table. Requires `database_name` and `table_name` — call `list_databases` and `list_tables` first.
23
24<usecase>
25ALWAYS call this before writing queries to understand:
26- Column names and data types
27- Which columns are nullable, primary keys, or have defaults
28- Foreign key relationships for writing JOINs
29</usecase>
30
31<examples>
32✓ "What columns does the orders table have?" → get_table_schema(database_name="mydb", table_name="orders")
33✓ Before writing a SELECT → get_table_schema first to confirm column names
34✓ "How are users and orders related?" → check foreign keys in both tables
35</examples>
36
37<what_it_returns>
38A JSON object with table_name and columns keyed by column name, each containing type, nullable, key, default, and foreign_key info.
39</what_it_returns>"#;
40}
41
42impl ToolBase for GetTableSchemaTool {
43    type Parameter = GetTableSchemaRequest;
44    type Output = TableSchemaResponse;
45    type Error = ErrorData;
46
47    fn name() -> Cow<'static, str> {
48        Self::NAME.into()
49    }
50
51    fn title() -> Option<String> {
52        Some(Self::TITLE.into())
53    }
54
55    fn description() -> Option<Cow<'static, str>> {
56        Some(Self::DESCRIPTION.into())
57    }
58
59    fn annotations() -> Option<ToolAnnotations> {
60        Some(
61            ToolAnnotations::new()
62                .read_only(true)
63                .destructive(false)
64                .idempotent(true)
65                .open_world(false),
66        )
67    }
68}
69
70impl AsyncTool<PostgresHandler> for GetTableSchemaTool {
71    async fn invoke(handler: &PostgresHandler, params: Self::Parameter) -> Result<Self::Output, Self::Error> {
72        Ok(handler.get_table_schema(&params).await?)
73    }
74}
75
76impl PostgresHandler {
77    /// Returns column definitions with foreign key relationships.
78    ///
79    /// # Errors
80    ///
81    /// Returns [`AppError`] if validation fails or the query errors.
82    pub async fn get_table_schema(&self, request: &GetTableSchemaRequest) -> Result<TableSchemaResponse, AppError> {
83        let table = &request.table_name;
84        validate_identifier(table)?;
85        let db = if request.database_name.is_empty() {
86            None
87        } else {
88            Some(request.database_name.as_str())
89        };
90
91        // 1. Get basic schema
92        let schema_sql = format!(
93            "SELECT column_name, data_type, is_nullable, column_default, \
94                    character_maximum_length \
95             FROM information_schema.columns \
96             WHERE table_schema = 'public' AND table_name = '{table}' \
97             ORDER BY ordinal_position"
98        );
99        let rows = self.connection.fetch(&schema_sql, db).await?;
100
101        if rows.is_empty() {
102            return Err(AppError::TableNotFound(table.clone()));
103        }
104
105        let mut columns: HashMap<String, Value> = HashMap::new();
106        for row in &rows {
107            let col_name = row
108                .get("column_name")
109                .and_then(Value::as_str)
110                .unwrap_or_default()
111                .to_owned();
112            let data_type = row
113                .get("data_type")
114                .and_then(Value::as_str)
115                .unwrap_or_default()
116                .to_owned();
117            let nullable = row
118                .get("is_nullable")
119                .and_then(Value::as_str)
120                .unwrap_or_default()
121                .to_owned();
122            let default = row.get("column_default").and_then(Value::as_str).map(str::to_owned);
123            columns.insert(
124                col_name,
125                json!({
126                    "type": data_type,
127                    "nullable": nullable.to_uppercase() == "YES",
128                    "key": Value::Null,
129                    "default": default,
130                    "extra": Value::Null,
131                    "foreign_key": Value::Null,
132                }),
133            );
134        }
135
136        // 2. Get FK relationships
137        let fk_sql = format!(
138            "SELECT \
139                kcu.column_name, \
140                tc.constraint_name, \
141                ccu.table_name AS referenced_table, \
142                ccu.column_name AS referenced_column, \
143                rc.update_rule AS on_update, \
144                rc.delete_rule AS on_delete \
145            FROM information_schema.table_constraints tc \
146            JOIN information_schema.key_column_usage kcu \
147                ON tc.constraint_name = kcu.constraint_name \
148                AND tc.table_schema = kcu.table_schema \
149            JOIN information_schema.constraint_column_usage ccu \
150                ON ccu.constraint_name = tc.constraint_name \
151                AND ccu.table_schema = tc.table_schema \
152            JOIN information_schema.referential_constraints rc \
153                ON rc.constraint_name = tc.constraint_name \
154                AND rc.constraint_schema = tc.table_schema \
155            WHERE tc.constraint_type = 'FOREIGN KEY' \
156                AND tc.table_name = '{table}' \
157                AND tc.table_schema = 'public'"
158        );
159        let fk_rows = self.connection.fetch(&fk_sql, db).await?;
160
161        for fk_row in &fk_rows {
162            let col_name = fk_row
163                .get("column_name")
164                .and_then(Value::as_str)
165                .unwrap_or_default()
166                .to_owned();
167            if let Some(col_info) = columns.get_mut(&col_name)
168                && let Some(obj) = col_info.as_object_mut()
169            {
170                obj.insert(
171                    "foreign_key".to_string(),
172                    json!({
173                        "constraint_name": fk_row.get("constraint_name").and_then(Value::as_str),
174                        "referenced_table": fk_row.get("referenced_table").and_then(Value::as_str),
175                        "referenced_column": fk_row.get("referenced_column").and_then(Value::as_str),
176                        "on_update": fk_row.get("on_update").and_then(Value::as_str),
177                        "on_delete": fk_row.get("on_delete").and_then(Value::as_str),
178                    }),
179                );
180            }
181        }
182
183        Ok(TableSchemaResponse {
184            table_name: table.clone(),
185            columns: json!(columns),
186        })
187    }
188}
database_mcp_postgres/tools/get_table_schema.rs

database_mcp_postgres/tools/
get_table_schema.rs
