Laravel migration - Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails

问题

I am trying to run a migration for a table inventories that I have created with this migration:

Schema::create('inventories', function (Blueprint $table) {
    $table->increments('id');
    $table->integer('remote_id')->unsigned();
    $table->integer('local_id')->unsigned();
    $table->string('local_type');
    $table->string('url')->nullable()->unique();
    $table->timestamps();
});

I am trying to add a run a migration where I am adding a foreign key to the table:

Schema::table('inventories', function (Blueprint $table) {
    $table->foreign('local_id')->references('id')->on('contents')->onDelete('cascade');
});

But, I am getting an error when I try to run the migration:

[Illuminate\Database\QueryException]

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (middleton
.#sql-5d6_162a, CONSTRAINT inventories_local_id_foreign FOREIGN KEY (local_id) REFERENCES contents (id) ON DELETE CASCADE ) (SQL: alter table inventories add constraint inventories_local_id_foreign foreign key (local_id) references contents (id) on delete cascade)

[PDOException]

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (middleton
.#sql-5d6_162a, CONSTRAINT inventories_local_id_foreign FOREIGN KEY (local_id) REFERENCES contents (id) ON DELETE CASCADE )

What am I doing wrong?

回答1:

You probably have some records in the inventories table with local_id that does not have corresponding id in the contents table, hence the error. You could solve it by one of the two ways:

• Run the migration with foreign_key_checks turned off. This will disabled the foreign key constraints for the existing rows (if that's what you want). It's documented here
• Insert only those records that have corresponding id field in contents table. You can use INSERT INTO.. WHERE EXISTS query to filter the records out, and insert only those records.

回答2:

had the same problem. fixed it by adding nullable to field

Schema::create('table_name', function (Blueprint $table) {
    ...
    $table->integer('some_id')->unsigned()->nullable();
    $table->foreign('some_id')->references('id')->on('other_table');
    ...
});

note that after migration all existed rows will have some_id = NULL

来源：https://stackoverflow.com/questions/47266089/laravel-migration-integrity-constraint-violation-1452-cannot-add-or-update-a