laravel 数据库 Migrations详解

• Introduction
• Generating Migrations
• Migration Structure
• Running Migrations
  • Rolling Back Migrations
• Tables
  • Creating Tables
  • Renaming / Dropping Tables
• Columns
  • Creating Columns
  • Column Modifiers
  • Modifying Columns
  • Dropping Columns
• Indexes
  • Creating Indexes
  • Dropping Indexes
  • Foreign Key Constraints

Introduction

Migrations are like version control for your database, allowing your team to easily modify and share the application’s database schema. Migrations are typically paired with Laravel’s schema builder to easily build your application’s database schema. If you have ever had to tell a teammate to manually add a column to their local database schema, you’ve faced the problem that database migrations solve.

The Laravel Schema facade provides database agnostic support for creating and manipulating tables across all of Laravel’s supported database systems.

Generating Migrations

To create a migration, use the make:migration Artisan command:

php artisan make:migration create_users_table

The new migration will be placed in your database/migrations directory. Each migration file name contains a timestamp which allows Laravel to determine the order of the migrations.

The --table and --create options may also be used to indicate the name of the table and whether the migration will be creating a new table. These options simply pre-fill the generated migration stub file with the specified table:

php artisan make:migration create_users_table --create=users

php artisan make:migration add_votes_to_users_table --table=users

If you would like to specify a custom output path for the generated migration, you may use the --path option when executing the make:migration command. The given path should be relative to your application’s base path.

Migration Structure

A migration class contains two methods: up and down. The up method is used to add new tables, columns, or indexes to your database, while the down method should simply reverse the operations performed by the up method.

Within both of these methods you may use the Laravel schema builder to expressively create and modify tables. To learn about all of the methods available on the Schema builder, check out its documentation. For example, this migration example creates a flights table:

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateFlightsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('flights', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->string('airline');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('flights');
    }
}

Running Migrations

To run all of your outstanding migrations, execute the migrate Artisan command:

php artisan migrate

If you are using the Homestead virtual machine, you should run this command from within your virtual machine.

Forcing Migrations To Run In Production

Some migration operations are destructive, which means they may cause you to lose data. In order to protect you from running these commands against your production database, you will be prompted for confirmation before the commands are executed. To force the commands to run without a prompt, use the --force flag:

php artisan migrate --force

Rolling Back Migrations

To rollback the latest migration operation, you may use the rollback command. This command rolls back the last “batch” of migrations, which may include multiple migration files:

php artisan migrate:rollback

You may rollback a limited number of migrations by providing the step option to the rollbackcommand. For example, the following command will rollback the last five migrations:

php artisan migrate:rollback --step=5

The migrate:reset command will roll back all of your application’s migrations:

php artisan migrate:reset

Rollback & Migrate In Single Command

The migrate:refresh command will roll back all of your migrations and then execute the migrate command. This command effectively re-creates your entire database:

php artisan migrate:refresh

// Refresh the database and run all database seeds...
php artisan migrate:refresh --seed

You may rollback & re-migrate a limited number of migrations by providing the step option to the refresh command. For example, the following command will rollback & re-migrate the last five migrations:

php artisan migrate:refresh --step=5

Tables

Creating Tables

To create a new database table, use the create method on the Schema facade. The createmethod accepts two arguments. The first is the name of the table, while the second is a Closurewhich receives a Blueprint object that may be used to define the new table:

Schema::create('users', function (Blueprint $table) {
    $table->increments('id');
});

Of course, when creating the table, you may use any of the schema builder’s column methods to define the table’s columns.

Checking For Table / Column Existence

You may easily check for the existence of a table or column using the hasTable and hasColumnmethods:

if (Schema::hasTable('users')) {
    //
}

if (Schema::hasColumn('users', 'email')) {
    //
}

Connection & Storage Engine

If you want to perform a schema operation on a database connection that is not your default connection, use the connection method:

Schema::connection('foo')->create('users', function (Blueprint $table) {
    $table->increments('id');
});

You may use the engine property on the schema builder to define the table’s storage engine:

Schema::create('users', function (Blueprint $table) {
    $table->engine = 'InnoDB';

    $table->increments('id');
});

Renaming / Dropping Tables

To rename an existing database table, use the rename method:

Schema::rename($from, $to);

To drop an existing table, you may use the drop or dropIfExists methods:

Schema::drop('users');

Schema::dropIfExists('users');

Renaming Tables With Foreign Keys

Before renaming a table, you should verify that any foreign key constraints on the table have an explicit name in your migration files instead of letting Laravel assign a convention based name. Otherwise, the foreign key constraint name will refer to the old table name.

Columns

Creating Columns

The table method on the Schema facade may be used to update existing tables. Like the createmethod, the table method accepts two arguments: the name of the table and a Closure that receives a Blueprint instance you may use to add columns to the table:

Schema::table('users', function (Blueprint $table) {
    $table->string('email');
});

Available Column Types

Of course, the schema builder contains a variety of column types that you may specify when building your tables:

Column Modifiers

In addition to the column types listed above, there are several column “modifiers” you may use while adding a column to a database table. For example, to make the column “nullable”, you may use the nullable method:

Schema::table('users', function (Blueprint $table) {
    $table->string('email')->nullable();
});

Below is a list of all the available column modifiers. This list does not include the index modifiers:

Modifying Columns

Prerequisites

Before modifying a column, be sure to add the doctrine/dbal dependency to your composer.json file. The Doctrine DBAL library is used to determine the current state of the column and create the SQL queries needed to make the specified adjustments to the column:

composer require doctrine/dbal

Updating Column Attributes

The change method allows you to modify some existing column types to a new type or modify the column’s attributes. For example, you may wish to increase the size of a string column. To see the change method in action, let’s increase the size of the name column from 25 to 50:

Schema::table('users', function (Blueprint $table) {
    $table->string('name', 50)->change();
});

We could also modify a column to be nullable:

Schema::table('users', function (Blueprint $table) {
    $table->string('name', 50)->nullable()->change();
});

The following column types can not be “changed”: char, double, enum, mediumInteger, timestamp, tinyInteger, ipAddress, json, jsonb, macAddress, mediumIncrements, morphs, nullableMorphs, nullableTimestamps, softDeletes, timeTz, timestampTz, timestamps, timestampsTz, unsignedMediumInteger, unsignedTinyInteger, uuid.

Renaming Columns

To rename a column, you may use the renameColumn method on the Schema builder. Before renaming a column, be sure to add the doctrine/dbal dependency to your composer.json file:

Schema::table('users', function (Blueprint $table) {
    $table->renameColumn('from', 'to');
});

Renaming any column in a table that also has a column of type enum is not currently supported.

Dropping Columns

To drop a column, use the dropColumn method on the Schema builder. Before dropping columns from a SQLite database, you will need to add the doctrine/dbal dependency to your composer.json file and run the composer update command in your terminal to install the library:

Schema::table('users', function (Blueprint $table) {
    $table->dropColumn('votes');
});

You may drop multiple columns from a table by passing an array of column names to the dropColumn method:

Schema::table('users', function (Blueprint $table) {
    $table->dropColumn(['votes', 'avatar', 'location']);
});

Dropping or modifying multiple columns within a single migration while using a SQLite database is not supported.

Indexes

Creating Indexes

The schema builder supports several types of indexes. First, let’s look at an example that specifies a column’s values should be unique. To create the index, we can simply chain the unique method onto the column definition:

$table->string('email')->unique();

Alternatively, you may create the index after defining the column. For example:

$table->unique('email');

You may even pass an array of columns to an index method to create a compound index:

$table->index(['account_id', 'created_at']);

Laravel will automatically generate a reasonable index name, but you may pass a second argument to the method to specify the name yourself:

$table->index('email', 'my_index_name');

Available Index Types

Index Lengths & MySQL / MariaDB

Laravel uses the utf8mb4 character set by default, which includes support for storing “emojis” in the database. If you are running a version of MySQL older than the 5.7.7 release or MariaDB older than the 10.2.2 release, you may need to manually configure the default string length generated by migrations in order for MySQL to create indexes for them. You may configure this by calling the Schema::defaultStringLength method within your AppServiceProvider:

use Illuminate\Support\Facades\Schema;

/**
 * Bootstrap any application services.
 *
 * @return void
 */
public function boot()
{
    Schema::defaultStringLength(191);
}

Alternatively, you may enable the innodb_large_prefix option for your database. Refer to your database’s documentation for instructions on how to properly enable this option.

Dropping Indexes

To drop an index, you must specify the index’s name. By default, Laravel automatically assigns a reasonable name to the indexes. Simply concatenate the table name, the name of the indexed column, and the index type. Here are some examples:

If you pass an array of columns into a method that drops indexes, the conventional index name will be generated based on the table name, columns and key type:

Schema::table('geo', function (Blueprint $table) {
    $table->dropIndex(['state']); // Drops index 'geo_state_index'
});

Foreign Key Constraints

Laravel also provides support for creating foreign key constraints, which are used to force referential integrity at the database level. For example, let’s define a user_id column on the posts table that references the id column on a users table:

Schema::table('posts', function (Blueprint $table) {
    $table->integer('user_id')->unsigned();

    $table->foreign('user_id')->references('id')->on('users');
});

You may also specify the desired action for the “on delete” and “on update” properties of the constraint:

$table->foreign('user_id')
      ->references('id')->on('users')
      ->onDelete('cascade');

To drop a foreign key, you may use the dropForeign method. Foreign key constraints use the same naming convention as indexes. So, we will concatenate the table name and the columns in the constraint then suffix the name with “_foreign”:

$table->dropForeign('posts_user_id_foreign');

Or, you may pass an array value which will automatically use the conventional constraint name when dropping:

$table->dropForeign(['user_id']);

You may enable or disable foreign key constraints within your migrations by using the following methods:

Schema::enableForeignKeyConstraints();

Schema::disableForeignKeyConstraints();