feat(migration): enhance primary key handling and add GIN index support in migration writer

pkg/writers/pgsql/writer_test.go

@@ -673,9 +673,14 @@ func TestPrimaryKeyExistenceCheck(t *testing.T) {
 		t.Errorf("Output missing logic to drop auto-generated primary key\nFull output:\n%s", output)
 	}
 
-	// Verify it checks for our specific named constraint before adding it
-	if !strings.Contains(output, "constraint_name = 'pk_public_products'") {
-		t.Errorf("Output missing check for our named primary key constraint\nFull output:\n%s", output)
+	// Verify it compares the current primary key columns before dropping/recreating
+	if !strings.Contains(output, "current_pk_matches") || !strings.Contains(output, "ARRAY['id']") {
+		t.Errorf("Output missing safe primary key comparison logic\nFull output:\n%s", output)
+	}
+
+	// Verify it only adds the desired key when no PK exists or an auto-generated mismatch was dropped
+	if !strings.Contains(output, "current_pk_name IS NULL") || !strings.Contains(output, "current_pk_name IN ('products_pkey', 'public_products_pkey')") {
+		t.Errorf("Output missing guarded primary key creation logic\nFull output:\n%s", output)
 	}
 }
 
@@ -766,6 +771,43 @@ func TestColumnSizeSpecifiers(t *testing.T) {
 	}
 }
 
+func TestWriteDatabase_PrimaryKeyTemplateDoesNotDropMatchingAutoPrimaryKey(t *testing.T) {
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+	table := models.InitTable("learnings", "public")
+
+	idCol := models.InitColumn("id", "learnings", "public")
+	idCol.Type = "bigint"
+	idCol.IsPrimaryKey = true
+	table.Columns["id"] = idCol
+
+	parentCol := models.InitColumn("duplicate_of_learning_id", "learnings", "public")
+	parentCol.Type = "bigint"
+	table.Columns["duplicate_of_learning_id"] = parentCol
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	var buf bytes.Buffer
+	writer := NewWriter(&writers.WriterOptions{})
+	writer.writer = &buf
+
+	if err := writer.WriteDatabase(db); err != nil {
+		t.Fatalf("WriteDatabase failed: %v", err)
+	}
+
+	output := buf.String()
+	if !strings.Contains(output, "current_pk_matches") {
+		t.Fatalf("expected generated SQL to compare current PK columns, got:\n%s", output)
+	}
+	if !strings.Contains(output, "ARRAY['id']") {
+		t.Fatalf("expected generated SQL to compare against desired PK columns, got:\n%s", output)
+	}
+	if !strings.Contains(output, "NOT current_pk_matches") {
+		t.Fatalf("expected generated SQL to avoid dropping matching PKs, got:\n%s", output)
+	}
+}
+
 func TestGenerateColumnDefinition_PreservesExplicitTypeModifiers(t *testing.T) {
 	writer := NewWriter(&writers.WriterOptions{})