Add a SqlGenerator

Prerequisites

Implementing Liquibase extensions requires an understanding of Java. You will be creating classes, overriding methods, and working with inheritance hierarchies.

Project Setup

If you have not already created a repository to hold your code, see Your First Extension in the Getting Started guide.

Overview

When defining the execution logic for a particular SqlGenerator, the interface you are going to implement is liquibase.sqlgenerator.SqlGenerator.

Best Practices

Base Class

There is a liquibase.sqlgenerator.core.AbstractSqlGenerator base class you can use which limits the number of methods you must implement.

Subclassing

When adding a SqlGenerator that provides database-specific logic, it's often best (but not required) to subclass the default SqlGenerator for that SqlStatement and override and change only the specific parts that need to be different while preserving the rest of the logic.

Also, remember to always check the parent class's signature for methods it defines. It may have methods like nullComesBeforeType() which it uses to determine the final SQL and you simply have to override that method to get the correct SQL.

Instanceof

Specifying database-specific functionality is best done with if (database instanceof ExampleDatabase) blocks around the database-specific logic.

Using "instanceof" rather than "equals" allows the logic to apply to any subclasses (variants) of the given database.

New Operations

If you are looking to define a new operation, create a new SqlStatement class and then the SqlGenerator for it.

Extending SqlGenerator

When extending an existing SqlGenerator class, you must override getPriority() so it will be picked over the base implementation you are extending. If you do not override this function they will both return the same priority and one will be chosen at random.

To ensure a higher value is returned, return super.getPriority() + 5.

API Documentation

A complete description of the API, including what methods must be implemented and how is available on the liquibase.sqlgenerator.SqlGenerator API page.

Example Code

package com.example.sqlgenerator;

import com.example.database.ExampleDatabase;
import liquibase.database.Database;
import liquibase.exception.ValidationErrors;
import liquibase.sql.Sql;
import liquibase.sql.UnparsedSql;
import liquibase.sqlgenerator.SqlGeneratorChain;
import liquibase.sqlgenerator.core.AbstractSqlGenerator;
import liquibase.statement.core.RenameColumnStatement;
import liquibase.structure.core.Column;

public class RenameColumnGeneratorExample extends AbstractSqlGenerator<RenameColumnStatement> {
    @Override
    public int getPriority() {
        return PRIORITY_DATABASE;
    }

    @Override
    public boolean supports(RenameColumnStatement statement, Database database) {
        return database instanceof ExampleDatabase;
    }

    @Override
    public ValidationErrors validate(RenameColumnStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        ValidationErrors errors = new ValidationErrors();
        errors.checkRequiredField("tableName", statement.getTableName());
        errors.checkRequiredField("oldColumnName", statement.getOldColumnName());
        errors.checkRequiredField("newColumnName", statement.getNewColumnName());

        return errors;
    }

    @Override
    public Sql[] generateSql(RenameColumnStatement statement, Database database, SqlGeneratorChain sqlGeneratorChain) {
        return new Sql[]{
                new UnparsedSql("alter table " +
                        database.escapeTableName(statement.getCatalogName(), statement.getSchemaName(), statement.getTableName()) +
                        " rename column " +
                        database.escapeObjectName(statement.getOldColumnName(), Column.class) +
                        " to " +
                        database.escapeObjectName(statement.getNewColumnName(), Column.class))
        };
    }
}