fix(pgsql-reader): pair composite foreign key columns by position

key_column_usage and constraint_column_usage were joined on constraint name only,
so an N-column foreign key returned N*N column pairs. Read from pg_constraint with
unnest(conkey, confkey) instead.

pkg/readers/pgsql/reader_test.go

@@ -1,10 +1,14 @@
 package pgsql
 
 import (
+	"context"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 
+	"github.com/jackc/pgx/v5"
+
 	"git.warky.dev/wdevs/relspecgo/pkg/models"
 	"git.warky.dev/wdevs/relspecgo/pkg/readers"
 )
@@ -499,3 +503,60 @@ func TestMapDataType_ExtensionTypesPreserveModifiers(t *testing.T) {
 		})
 	}
 }
+
+func TestReader_CompositeForeignKeyColumnsArePairedOnce(t *testing.T) {
+	connStr := getTestConnectionString(t)
+
+	ctx := context.Background()
+	conn, err := pgx.Connect(ctx, connStr)
+	if err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer conn.Close(ctx)
+
+	const schema = "relspec_fk_test"
+	setup := []string{
+		"DROP SCHEMA IF EXISTS " + schema + " CASCADE",
+		"CREATE SCHEMA " + schema,
+		"CREATE TABLE " + schema + ".parent (a int, b int, PRIMARY KEY (a, b))",
+		"CREATE TABLE " + schema + ".child (x int, y int, CONSTRAINT fk_child_parent FOREIGN KEY (x, y) REFERENCES " + schema + ".parent (a, b) ON DELETE CASCADE)",
+	}
+	for _, stmt := range setup {
+		if _, err := conn.Exec(ctx, stmt); err != nil {
+			t.Fatalf("setup %q: %v", stmt, err)
+		}
+	}
+	defer conn.Exec(ctx, "DROP SCHEMA IF EXISTS "+schema+" CASCADE")
+
+	reader := NewReader(&readers.ReaderOptions{ConnectionString: connStr})
+	db, err := reader.ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase: %v", err)
+	}
+
+	for _, s := range db.Schemas {
+		if s.Name != schema {
+			continue
+		}
+		for _, tbl := range s.Tables {
+			if tbl.Name != "child" {
+				continue
+			}
+			fk := tbl.Constraints["fk_child_parent"]
+			if fk == nil {
+				t.Fatal("foreign key fk_child_parent not read")
+			}
+			if got := strings.Join(fk.Columns, ","); got != "x,y" {
+				t.Errorf("columns = %q, want x,y", got)
+			}
+			if got := strings.Join(fk.ReferencedColumns, ","); got != "a,b" {
+				t.Errorf("referenced columns = %q, want a,b", got)
+			}
+			if fk.OnDelete != "CASCADE" || fk.OnUpdate != "NO ACTION" {
+				t.Errorf("rules = %s/%s, want CASCADE/NO ACTION", fk.OnDelete, fk.OnUpdate)
+			}
+			return
+		}
+	}
+	t.Fatal("test schema/table not found in read result")
+}