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.go

@@ -320,6 +320,31 @@ func (w *Writer) GenerateSchemaStatements(schema *models.Schema) ([]string, erro
 		}
 	}
 
+	// Phase 5.7: Check constraints
+	for _, table := range schema.Tables {
+		for _, constraint := range table.Constraints {
+			if constraint.Type != models.CheckConstraint {
+				continue
+			}
+
+			// Wrap in DO block to check for existing constraint
+			stmt := fmt.Sprintf("DO $$\nBEGIN\n"+
+				"  IF NOT EXISTS (\n"+
+				"    SELECT 1 FROM information_schema.table_constraints\n"+
+				"    WHERE table_schema = '%s'\n"+
+				"      AND table_name = '%s'\n"+
+				"      AND constraint_name = '%s'\n"+
+				"  ) THEN\n"+
+				"    ALTER TABLE %s.%s ADD CONSTRAINT %s CHECK (%s);\n"+
+				"  END IF;\n"+
+				"END;\n$$",
+				schema.Name, table.Name, constraint.Name,
+				schema.SQLName(), table.SQLName(), constraint.Name,
+				constraint.Expression)
+			statements = append(statements, stmt)
+		}
+	}
+
 	// Phase 6: Foreign keys
 	for _, table := range schema.Tables {
 		for _, constraint := range table.Constraints {
@@ -572,6 +597,11 @@ func (w *Writer) WriteSchema(schema *models.Schema) error {
 		return err
 	}
 
+	// Phase 5.7: Create check constraints (priority 190)
+	if err := w.writeCheckConstraints(schema); err != nil {
+		return err
+	}
+
 	// Phase 6: Create foreign key constraints (priority 195)
 	if err := w.writeForeignKeys(schema); err != nil {
 		return err
@@ -944,6 +974,48 @@ func (w *Writer) writeUniqueConstraints(schema *models.Schema) error {
 	return nil
 }
 
+// writeCheckConstraints generates ALTER TABLE statements for check constraints
+func (w *Writer) writeCheckConstraints(schema *models.Schema) error {
+	fmt.Fprintf(w.writer, "-- Check constraints for schema: %s\n", schema.Name)
+
+	for _, table := range schema.Tables {
+		// Sort constraints by name for consistent output
+		constraintNames := make([]string, 0, len(table.Constraints))
+		for name, constraint := range table.Constraints {
+			if constraint.Type == models.CheckConstraint {
+				constraintNames = append(constraintNames, name)
+			}
+		}
+		sort.Strings(constraintNames)
+
+		for _, name := range constraintNames {
+			constraint := table.Constraints[name]
+
+			// Skip if expression is empty
+			if constraint.Expression == "" {
+				continue
+			}
+
+			// Wrap in DO block to check for existing constraint
+			fmt.Fprintf(w.writer, "DO $$\n")
+			fmt.Fprintf(w.writer, "BEGIN\n")
+			fmt.Fprintf(w.writer, "  IF NOT EXISTS (\n")
+			fmt.Fprintf(w.writer, "    SELECT 1 FROM information_schema.table_constraints\n")
+			fmt.Fprintf(w.writer, "    WHERE table_schema = '%s'\n", schema.Name)
+			fmt.Fprintf(w.writer, "      AND table_name = '%s'\n", table.Name)
+			fmt.Fprintf(w.writer, "      AND constraint_name = '%s'\n", constraint.Name)
+			fmt.Fprintf(w.writer, "  ) THEN\n")
+			fmt.Fprintf(w.writer, "    ALTER TABLE %s.%s ADD CONSTRAINT %s CHECK (%s);\n",
+				schema.SQLName(), table.SQLName(), constraint.Name, constraint.Expression)
+			fmt.Fprintf(w.writer, "  END IF;\n")
+			fmt.Fprintf(w.writer, "END;\n")
+			fmt.Fprintf(w.writer, "$$;\n\n")
+		}
+	}
+
+	return nil
+}
+
 // writeForeignKeys generates ALTER TABLE statements for foreign keys
 func (w *Writer) writeForeignKeys(schema *models.Schema) error {
 	fmt.Fprintf(w.writer, "-- Foreign keys for schema: %s\n", schema.Name)
@@ -1040,6 +1112,84 @@ func (w *Writer) writeForeignKeys(schema *models.Schema) error {
 			fmt.Fprintf(w.writer, "  END IF;\n")
 			fmt.Fprintf(w.writer, "END;\n$$;\n\n")
 		}
+
+		// Also process any foreign key constraints that don't have a relationship
+		processedConstraints := make(map[string]bool)
+		for _, rel := range table.Relationships {
+			fkName := rel.ForeignKey
+			if fkName == "" {
+				fkName = rel.Name
+			}
+			if fkName != "" {
+				processedConstraints[fkName] = true
+			}
+		}
+
+		// Find unprocessed foreign key constraints
+		constraintNames := make([]string, 0)
+		for name, constraint := range table.Constraints {
+			if constraint.Type == models.ForeignKeyConstraint && !processedConstraints[name] {
+				constraintNames = append(constraintNames, name)
+			}
+		}
+		sort.Strings(constraintNames)
+
+		for _, name := range constraintNames {
+			constraint := table.Constraints[name]
+
+			// Build column lists
+			sourceColumns := make([]string, 0, len(constraint.Columns))
+			for _, colName := range constraint.Columns {
+				if col, ok := table.Columns[colName]; ok {
+					sourceColumns = append(sourceColumns, col.SQLName())
+				} else {
+					sourceColumns = append(sourceColumns, colName)
+				}
+			}
+
+			targetColumns := make([]string, 0, len(constraint.ReferencedColumns))
+			for _, colName := range constraint.ReferencedColumns {
+				targetColumns = append(targetColumns, strings.ToLower(colName))
+			}
+
+			if len(sourceColumns) == 0 || len(targetColumns) == 0 {
+				continue
+			}
+
+			onDelete := "NO ACTION"
+			if constraint.OnDelete != "" {
+				onDelete = strings.ToUpper(constraint.OnDelete)
+			}
+
+			onUpdate := "NO ACTION"
+			if constraint.OnUpdate != "" {
+				onUpdate = strings.ToUpper(constraint.OnUpdate)
+			}
+
+			refSchema := constraint.ReferencedSchema
+			if refSchema == "" {
+				refSchema = schema.Name
+			}
+			refTable := constraint.ReferencedTable
+
+			// Use DO block to check if constraint exists before adding
+			fmt.Fprintf(w.writer, "DO $$\nBEGIN\n")
+			fmt.Fprintf(w.writer, "  IF NOT EXISTS (\n")
+			fmt.Fprintf(w.writer, "    SELECT 1 FROM information_schema.table_constraints\n")
+			fmt.Fprintf(w.writer, "    WHERE table_schema = '%s'\n", schema.Name)
+			fmt.Fprintf(w.writer, "      AND table_name = '%s'\n", table.Name)
+			fmt.Fprintf(w.writer, "      AND constraint_name = '%s'\n", constraint.Name)
+			fmt.Fprintf(w.writer, "  ) THEN\n")
+			fmt.Fprintf(w.writer, "    ALTER TABLE %s.%s\n", schema.SQLName(), table.SQLName())
+			fmt.Fprintf(w.writer, "      ADD CONSTRAINT %s\n", constraint.Name)
+			fmt.Fprintf(w.writer, "      FOREIGN KEY (%s)\n", strings.Join(sourceColumns, ", "))
+			fmt.Fprintf(w.writer, "      REFERENCES %s.%s (%s)\n",
+				refSchema, refTable, strings.Join(targetColumns, ", "))
+			fmt.Fprintf(w.writer, "      ON DELETE %s\n", onDelete)
+			fmt.Fprintf(w.writer, "      ON UPDATE %s;\n", onUpdate)
+			fmt.Fprintf(w.writer, "  END IF;\n")
+			fmt.Fprintf(w.writer, "END;\n$$;\n\n")
+		}
 	}
 
 	return nil