Add constraint method to DatabaseError trait

Override `constraint` method for `PgDatabaseError`.

tests/postgres/postgres.rs

@@ -80,6 +80,42 @@ async fn it_can_inspect_errors() -> anyhow::Result<()> {
     assert_eq!(err.code(), "42703");
     assert_eq!(err.position(), Some(PgErrorPosition::Original(8)));
     assert_eq!(err.routine(), Some("errorMissingColumn"));
+    assert_eq!(err.constraint(), None);
+
+    Ok(())
+}
+
+#[sqlx_macros::test]
+async fn it_can_inspect_constraint_errors() -> anyhow::Result<()> {
+    let mut conn = new::<Postgres>().await?;
+
+    let res: Result<_, sqlx::Error> =
+        sqlx::query("INSERT INTO products VALUES (1, 'Product 1', 0);")
+            .execute(&mut conn)
+            .await;
+    let err = res.unwrap_err();
+
+    // can also do [as_database_error] or use `match ..`
+    let err = err.into_database_error().unwrap();
+
+    assert_eq!(
+        err.message(),
+        "new row for relation \"products\" violates check constraint \"products_price_check\""
+    );
+    assert_eq!(err.code().as_deref(), Some("23514"));
+
+    // can also do [downcast_ref]
+    let err: Box<PgDatabaseError> = err.downcast();
+
+    assert_eq!(err.severity(), PgSeverity::Error);
+    assert_eq!(
+        err.message(),
+        "new row for relation \"products\" violates check constraint \"products_price_check\""
+    );
+    assert_eq!(err.code(), "23514");
+    assert_eq!(err.position(), None);
+    assert_eq!(err.routine(), Some("ExecConstraints"));
+    assert_eq!(err.constraint(), Some("products_price_check"));
 
     Ok(())
 }

tests/postgres/setup.sql

@@ -23,3 +23,9 @@ CREATE TYPE float_range AS RANGE
     subtype = float8,
     subtype_diff = float8mi
 );
+
+CREATE TABLE products (
+    product_no INTEGER,
+    name TEXT,
+    price NUMERIC CHECK (price > 0)
+);