ドライバの作成

GORM offers built-in support for popular databases like SQLite, MySQL, Postgres, SQLServer, and ClickHouse. However, when you need to integrate GORM with databases that are not directly supported or have unique features, you can create a custom driver. This involves implementing the Dialector interface provided by GORM.

Compatibility with MySQL or Postgres Dialects

For databases that closely resemble the behavior of MySQL or Postgres, you can often use the respective dialects directly. However, if your database significantly deviates from these dialects or offers additional features, developing a custom driver is recommended.

Implementing the Dialector

The Dialector interface in GORM consists of methods that a database driver must implement to facilitate communication between the database and GORM. Let’s break down the key methods:

type Dialector interface {
  Name() string                      // Returns the name of the database dialect
  Initialize(*DB) error              // Initializes the database connection
  Migrator(db *DB) Migrator          // Provides the database migration tool
  DataTypeOf(*schema.Field) string   // Determines the data type for a schema field
  DefaultValueOf(*schema.Field) clause.Expression // Provides the default value for a schema field
  BindVarTo(writer clause.Writer, stmt *Statement, v interface{}) // Handles variable binding in SQL statements
  QuoteTo(clause.Writer, string)     // Manages quoting of identifiers
  Explain(sql string, vars ...interface{}) string // Formats SQL statements with variables
}

Each method in this interface serves a crucial role in how GORM interacts with the database, from establishing connections to handling queries and migrations.

Nested Transaction Support

If your database supports savepoints, you can implement the SavePointerDialectorInterface to get the Nested Transaction Support and SavePoint support.

type SavePointerDialectorInterface interface {
    SavePoint(tx *DB, name string) error // Saves a savepoint within a transaction
    RollbackTo(tx *DB, name string) error // Rolls back a transaction to the specified savepoint
}

By implementing these methods, you enable support for savepoints and nested transactions, offering advanced transaction management capabilities.

Custom Clause Builders

Defining custom clause builders in GORM allows you to extend the query capabilities for specific database operations. In this example, we’ll go through the steps to define a custom clause builder for the “LIMIT” clause, which may have database-specific behavior.

• Step 1: Define a Custom Clause Builder Function:

To create a custom clause builder, you need to define a function that adheres to the clause.ClauseBuilder interface. This function will be responsible for constructing the SQL clause for a specific operation. In our example, we’ll create a custom “LIMIT” clause builder.

Here’s the basic structure of a custom “LIMIT” clause builder function:

func MyCustomLimitBuilder(c clause.Clause, builder clause.Builder) {
    if limit, ok := c.Expression.(clause.Limit); ok {
        // Handle the "LIMIT" clause logic here
        // You can access the limit values using limit.Limit and limit.Offset
        builder.WriteString("MYLIMIT")
    }
}

• The function takes two parameters: c of type clause.Clause and builder of type clause.Builder.
• Inside the function, we check if the c.Expression is a clause.Limit. If it is, we proceed to handle the “LIMIT” clause logic.

Replace MYLIMIT with the actual SQL logic for your database. This is where you can implement database-specific behavior for the “LIMIT” clause.

• Step 2: Register the Custom Clause Builder:

To make your custom “LIMIT” clause builder available to GORM, register it with the db.ClauseBuilders map, typically during driver initialization. Here’s how to register the custom “LIMIT” clause builder:

func (d *MyDialector) Initialize(db *gorm.DB) error {
    // Register the custom "LIMIT" clause builder
    db.ClauseBuilders["LIMIT"] = MyCustomLimitBuilder

    //...
}

In this code, we use the key "LIMIT" to register our custom clause builder in the db.ClauseBuilders map, associating our custom builder with the “LIMIT” clause.

• Step 3: Use the Custom Clause Builder:

After registering the custom clause builder, GORM will call it when generating SQL statements that involve the “LIMIT” clause. You can use your custom logic to generate the SQL clause as needed.

Here’s an example of how you can use the custom “LIMIT” clause builder in a GORM query:

query := db.Model(&User{})

// Apply the custom "LIMIT" clause using the Limit method
query = query.Limit(10) // You can also provide an offset, e.g., query.Limit(10).Offset(5)

// Execute the query
result := query.Find(&results)
// SQL: SELECT * FROM users MYLIMIT

In this example, we use the Limit method with GORM, and behind the scenes, our custom “LIMIT” clause builder (MyCustomLimitBuilder) will be invoked to handle the generation of the “LIMIT” clause.

For inspiration and guidance, examining the MySQL Driver can be helpful. This driver demonstrates how the Dialector interface is implemented to suit the specific needs of the MySQL database.