marsdb 0.4.0

//! A small social graph -- the kind of query a relational database makes
//! awkward and a graph database makes direct: variable-length traversal,
//! and connecting two nodes that already exist. Run:
//!
//!     cargo run -p marsdb --example social_graph
//!
//! Writes social_graph.svg -- the full KNOWS network.

use marsdb::Database;

#[path = "common/viz.rs"]
mod viz;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let db = Database::in_memory()?;

    // One continuous chain, deliberately NOT comma-separated: every node
    // token CREATE sees always becomes a fresh node, *even* a repeated
    // variable name across comma-separated patterns in the same
    // statement -- writing `(a:Person {..Alice..})-[:KNOWS]->(b), (b)
    // -[:KNOWS]->(c)` looks like it reuses `b` but actually creates a
    // second, blank `b` instead (a real gap -- see the README's
    // Cypher-coverage section). A single unbroken chain like this one
    // sidesteps it -- each hop naturally continues from the node the
    // previous hop just created.
    db.execute(
        "CREATE (a:Person {name: 'Alice'})-[:KNOWS]->(b:Person {name: 'Bob'})\
                -[:KNOWS]->(c:Person {name: 'Carol'})\
                -[:KNOWS]->(d:Person {name: 'Dave'})",
    )?;
    db.execute("CREATE (:Person {name: 'Eve'})")?;

    // MATCH...CREATE is what connects nodes that already exist, which no
    // CREATE pattern (chained or not) can do: match `a`, carry it
    // through WITH, match the target, then CREATE reuses both bindings
    // instead of creating anything new. Alice ends up connected to Dave
    // two different ways (direct here, and via Bob then Carol) and to
    // Eve, who's otherwise unreachable from her.
    for target in ["Dave", "Eve"] {
        db.execute(&format!(
            "MATCH (a:Person {{name: 'Alice'}}) WITH a \
             MATCH (t:Person {{name: '{target}'}}) \
             CREATE (a)-[:KNOWS]->(t)"
        ))?;
    }

    let direct = db.execute("MATCH (:Person {name: 'Alice'})-[:KNOWS]->(f:Person) RETURN f.name")?;
    println!("Alice's direct friends: {}", names(&direct));

    // Variable-length traversal, 1 to 2 hops out. The executor's
    // variable-length expansion does a node-visited-set BFS per start
    // node, so Dave -- reachable two different ways -- only shows up
    // once here. No RETURN DISTINCT needed (MarsDB doesn't have that as
    // a general RETURN modifier yet, only inside an aggregate call like
    // count(DISTINCT x)).
    let network = db.execute("MATCH (:Person {name: 'Alice'})-[:KNOWS*1..2]->(f:Person) RETURN f.name")?;
    println!("Alice's network within 2 hops: {}", names(&network));

    // Who has the most outgoing KNOWS edges?
    let popular = db.execute(
        "MATCH (p:Person)-[:KNOWS]->(f:Person) \
         WITH p.name AS name, count(f) AS friends \
         RETURN name, friends ORDER BY friends DESC LIMIT 1",
    )?;
    if let Some(row) = popular.rows.first() {
        println!("Most connected: {} ({} friends)", show(&row[0]), show(&row[1]));
    }

    let nodes = ["Alice", "Bob", "Carol", "Dave", "Eve"];
    let all_edges = db.execute("MATCH (a:Person)-[:KNOWS]->(b:Person) RETURN a.name, b.name")?;
    let idx = |v: &marsdb::Value| -> usize {
        let name = show(v);
        nodes.iter().position(|n| *n == name).expect("known node")
    };
    let edges: Vec<(usize, usize)> = all_edges.rows.iter().map(|row| (idx(&row[0]), idx(&row[1]))).collect();
    viz::graph_viz("social_graph.svg", "Alice's social network", &nodes, &edges)?;
    println!("\nwrote social_graph.svg");

    Ok(())
}

fn names(result: &marsdb::QueryResult) -> String {
    result.rows.iter().map(|row| show(&row[0])).collect::<Vec<_>>().join(", ")
}

fn show(v: &marsdb::Value) -> String {
    match v {
        marsdb::Value::Property(marsdb::PropertyValue::String(s)) => s.clone(),
        marsdb::Value::Property(marsdb::PropertyValue::Int(i)) => i.to_string(),
        marsdb::Value::Property(marsdb::PropertyValue::Float(f)) => f.to_string(),
        marsdb::Value::Property(marsdb::PropertyValue::Bool(b)) => b.to_string(),
        marsdb::Value::Property(marsdb::PropertyValue::Null) | marsdb::Value::Null => "null".to_string(),
        other => format!("{other:?}"),
    }
}