feat(writer): 🎉 Add support for check constraints in schema generation

* Implement check constraints in the schema writer.
* Generate SQL statements to add check constraints if they do not exist.
* Add tests to verify correct generation of check constraints.

pkg/writers/pgsql/writer_test.go

@@ -234,6 +234,226 @@ func TestWriteUniqueConstraints(t *testing.T) {
 	}
 }
 
+func TestWriteCheckConstraints(t *testing.T) {
+	// Create a test database with check constraints
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	// Create table with check constraints
+	table := models.InitTable("products", "public")
+
+	// Add columns
+	priceCol := models.InitColumn("price", "products", "public")
+	priceCol.Type = "numeric(10,2)"
+	table.Columns["price"] = priceCol
+
+	statusCol := models.InitColumn("status", "products", "public")
+	statusCol.Type = "varchar(20)"
+	table.Columns["status"] = statusCol
+
+	quantityCol := models.InitColumn("quantity", "products", "public")
+	quantityCol.Type = "integer"
+	table.Columns["quantity"] = quantityCol
+
+	// Add check constraints
+	priceConstraint := &models.Constraint{
+		Name:       "ck_price_positive",
+		Type:       models.CheckConstraint,
+		Schema:     "public",
+		Table:      "products",
+		Expression: "price >= 0",
+	}
+	table.Constraints["ck_price_positive"] = priceConstraint
+
+	statusConstraint := &models.Constraint{
+		Name:       "ck_status_valid",
+		Type:       models.CheckConstraint,
+		Schema:     "public",
+		Table:      "products",
+		Expression: "status IN ('active', 'inactive', 'discontinued')",
+	}
+	table.Constraints["ck_status_valid"] = statusConstraint
+
+	quantityConstraint := &models.Constraint{
+		Name:       "ck_quantity_nonnegative",
+		Type:       models.CheckConstraint,
+		Schema:     "public",
+		Table:      "products",
+		Expression: "quantity >= 0",
+	}
+	table.Constraints["ck_quantity_nonnegative"] = quantityConstraint
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	// Create writer with output to buffer
+	var buf bytes.Buffer
+	options := &writers.WriterOptions{}
+	writer := NewWriter(options)
+	writer.writer = &buf
+
+	// Write the database
+	err := writer.WriteDatabase(db)
+	if err != nil {
+		t.Fatalf("WriteDatabase failed: %v", err)
+	}
+
+	output := buf.String()
+
+	// Print output for debugging
+	t.Logf("Generated SQL:\n%s", output)
+
+	// Verify check constraints are present
+	if !strings.Contains(output, "-- Check constraints for schema: public") {
+		t.Errorf("Output missing check constraints header")
+	}
+	if !strings.Contains(output, "ADD CONSTRAINT ck_price_positive CHECK (price >= 0)") {
+		t.Errorf("Output missing ck_price_positive check constraint\nFull output:\n%s", output)
+	}
+	if !strings.Contains(output, "ADD CONSTRAINT ck_status_valid CHECK (status IN ('active', 'inactive', 'discontinued'))") {
+		t.Errorf("Output missing ck_status_valid check constraint\nFull output:\n%s", output)
+	}
+	if !strings.Contains(output, "ADD CONSTRAINT ck_quantity_nonnegative CHECK (quantity >= 0)") {
+		t.Errorf("Output missing ck_quantity_nonnegative check constraint\nFull output:\n%s", output)
+	}
+}
+
+func TestWriteAllConstraintTypes(t *testing.T) {
+	// Create a comprehensive test with all constraint types
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	// Create orders table
+	ordersTable := models.InitTable("orders", "public")
+
+	// Add columns
+	idCol := models.InitColumn("id", "orders", "public")
+	idCol.Type = "integer"
+	idCol.IsPrimaryKey = true
+	ordersTable.Columns["id"] = idCol
+
+	userIDCol := models.InitColumn("user_id", "orders", "public")
+	userIDCol.Type = "integer"
+	userIDCol.NotNull = true
+	ordersTable.Columns["user_id"] = userIDCol
+
+	orderNumberCol := models.InitColumn("order_number", "orders", "public")
+	orderNumberCol.Type = "varchar(50)"
+	orderNumberCol.NotNull = true
+	ordersTable.Columns["order_number"] = orderNumberCol
+
+	totalCol := models.InitColumn("total", "orders", "public")
+	totalCol.Type = "numeric(10,2)"
+	ordersTable.Columns["total"] = totalCol
+
+	statusCol := models.InitColumn("status", "orders", "public")
+	statusCol.Type = "varchar(20)"
+	ordersTable.Columns["status"] = statusCol
+
+	// Add primary key constraint
+	pkConstraint := &models.Constraint{
+		Name:    "pk_orders",
+		Type:    models.PrimaryKeyConstraint,
+		Schema:  "public",
+		Table:   "orders",
+		Columns: []string{"id"},
+	}
+	ordersTable.Constraints["pk_orders"] = pkConstraint
+
+	// Add unique constraint
+	uniqueConstraint := &models.Constraint{
+		Name:    "uq_order_number",
+		Type:    models.UniqueConstraint,
+		Schema:  "public",
+		Table:   "orders",
+		Columns: []string{"order_number"},
+	}
+	ordersTable.Constraints["uq_order_number"] = uniqueConstraint
+
+	// Add check constraint
+	checkConstraint := &models.Constraint{
+		Name:       "ck_total_positive",
+		Type:       models.CheckConstraint,
+		Schema:     "public",
+		Table:      "orders",
+		Expression: "total > 0",
+	}
+	ordersTable.Constraints["ck_total_positive"] = checkConstraint
+
+	statusCheckConstraint := &models.Constraint{
+		Name:       "ck_status_valid",
+		Type:       models.CheckConstraint,
+		Schema:     "public",
+		Table:      "orders",
+		Expression: "status IN ('pending', 'completed', 'cancelled')",
+	}
+	ordersTable.Constraints["ck_status_valid"] = statusCheckConstraint
+
+	// Add foreign key constraint (referencing a users table)
+	fkConstraint := &models.Constraint{
+		Name:              "fk_orders_user",
+		Type:              models.ForeignKeyConstraint,
+		Schema:            "public",
+		Table:             "orders",
+		Columns:           []string{"user_id"},
+		ReferencedSchema:  "public",
+		ReferencedTable:   "users",
+		ReferencedColumns: []string{"id"},
+		OnDelete:          "CASCADE",
+		OnUpdate:          "CASCADE",
+	}
+	ordersTable.Constraints["fk_orders_user"] = fkConstraint
+
+	schema.Tables = append(schema.Tables, ordersTable)
+	db.Schemas = append(db.Schemas, schema)
+
+	// Create writer with output to buffer
+	var buf bytes.Buffer
+	options := &writers.WriterOptions{}
+	writer := NewWriter(options)
+	writer.writer = &buf
+
+	// Write the database
+	err := writer.WriteDatabase(db)
+	if err != nil {
+		t.Fatalf("WriteDatabase failed: %v", err)
+	}
+
+	output := buf.String()
+
+	// Print output for debugging
+	t.Logf("Generated SQL:\n%s", output)
+
+	// Verify all constraint types are present
+	expectedConstraints := map[string]string{
+		"Primary Key":      "PRIMARY KEY",
+		"Unique":           "ADD CONSTRAINT uq_order_number UNIQUE (order_number)",
+		"Check (total)":    "ADD CONSTRAINT ck_total_positive CHECK (total > 0)",
+		"Check (status)":   "ADD CONSTRAINT ck_status_valid CHECK (status IN ('pending', 'completed', 'cancelled'))",
+		"Foreign Key":      "FOREIGN KEY",
+	}
+
+	for name, expected := range expectedConstraints {
+		if !strings.Contains(output, expected) {
+			t.Errorf("Output missing %s constraint: %s\nFull output:\n%s", name, expected, output)
+		}
+	}
+
+	// Verify section headers
+	sections := []string{
+		"-- Primary keys for schema: public",
+		"-- Unique constraints for schema: public",
+		"-- Check constraints for schema: public",
+		"-- Foreign keys for schema: public",
+	}
+
+	for _, section := range sections {
+		if !strings.Contains(output, section) {
+			t.Errorf("Output missing section header: %s", section)
+		}
+	}
+}
+
 func TestWriteTable(t *testing.T) {
 	// Create a single table
 	table := models.InitTable("products", "public")