1. Why Doesn't Rails Generate A Foreign Key In The World
2. Why Doesn't Rails Generate A Foreign Key Examples
3. Why Doesn't Rails Generate A Foreign Key In Excel
4. Why Doesn't Rails Generate A Foreign Key In Spanish
5. Why Doesn't Rails Generate A Foreign Key Examples

SQL FOREIGN KEY Constraint

A FOREIGN KEY is a key used to link two tables together.

@derekprior we do the check for supportsindexesincreate? And you can make the same argument for that - if you're adding an index in the createtable block and your adapter doesn't support it then should it blow up? Anyway the problem is that td.foreignkeys is nil and not an empty array so it raises even if you've not passed foreignkey: true. Adds a new foreign key. Fromtable is the table with the key column, totable contains the referenced primary key. The foreign key will be named after the following pattern: fkrails.identifier is a 10 character long string which is deterministically generated from the fromtable and column.A custom name can be specified with the:name option. Yeetdba - find missing foreign key constraints yeetdba scans your rails tables for missing foreign key constraints. If there are no dangling records, it will create a migration to add the foreign key constraints on all the table it is safe. If you have dangling migrations, check the generator logs to see where you have invalid orphaned rows.

A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.

The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.

Look at the following two tables:

'Persons' table:

'Orders' table:

Notice that the 'PersonID' column in the 'Orders' table points to the 'PersonID' column in the 'Persons' table.

The 'PersonID' column in the 'Persons' table is the PRIMARY KEY in the 'Persons' table.

The 'PersonID' column in the 'Orders' table is a FOREIGN KEY in the 'Orders' table.

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

SQL FOREIGN KEY on CREATE TABLE

The following SQL creates a FOREIGN KEY on the 'PersonID' column when the 'Orders' table is created:

MySQL:

SQL Server / Oracle / MS Access:

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

SQL FOREIGN KEY on ALTER TABLE

To create a FOREIGN KEY constraint on the 'PersonID' column when the 'Orders' table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

SQL Server / Oracle / MS Access:

Summary: in this tutorial, you will learn about MySQL foreign key and how to create, drop, and disable a foreign key constraint.

Introduction to MySQL foreign key

A foreign key is a column or group of columns in a table that links to a column or group of columns in another table. The foreign key places constraints on data in the related tables, which allows MySQL to maintain referential integrity.

Let’s take a look at the following customers and orders tables from the sample database.

In this diagram, each customer can have zero or many orders and each order belongs to one customer.

The relationship between customers table and orders table is one-to-many. And this relationship is established by the foreign key in the orders table specified by the customerNumber column.

The customerNumber column in the orders table links to the customerNumber primary key column in the customers table.

The customers table is called the parent table or referenced table, and the orders table is known as the child table or referencing table.

Typically, the foreign key columns of the child table often refer to the primary key columns of the parent table.

A table can have more than one foreign key where each foreign key references to a primary key of the different parent tables.

Once a foreign key constraint is in place, the foreign key columns from the child table must have the corresponding row in the parent key columns of the parent table or values in these foreign key column must be NULL (see the SET NULL action example below).

For example, each row in the orders table has a customerNumber that exists in the customerNumber column of the customers table. Multiple rows in the orders table can have the same customerNumber.

Self-referencing foreign key

Sometimes, the child and parent tables may refer to the same table. In this case, the foreign key references back to the primary key within the same table.

See the following employees table from the sample database.

The reportTo column is a foreign key that refers to the employeeNumber column which is the primary key of the employees table.

This relationship allows the employees table to store the reporting structure between employees and managers. Each employee reports to zero or one employee and an employee can have zero or many subordinates.

Why Doesn't Rails Generate A Foreign Key In The World

The foreign key on the column reportTo is known as a recursive or self-referencing foreign key.

MySQL FOREIGN KEY syntax

Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement:

In this syntax:

First, specify the name of foreign key constraint that you want to create after the CONSTRAINT keyword. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint.

Why Doesn't Rails Generate A Foreign Key Examples

Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords. The foreign key name is also optional and is generated automatically if you skip it.

Third, specify the parent table followed by a list of comma-separated columns to which the foreign key columns reference.

Finally, specify how foreign key maintains the referential integrity between the child and parent tables by using the ON DELETE and ON UPDATE clauses. The reference_option determines action which MySQL will take when values in the parent key columns are deleted (ON DELETE) or updated (ON UPDATE).

MySQL has five reference options: CASCADE, SET NULL, NO ACTION, RESTRICT, and SET DEFAULT.

• CASCADE: if a row from the parent table is deleted or updated, the values of the matching rows in the child table automatically deleted or updated.
• SET NULL: if a row from the parent table is deleted or updated, the values of the foreign key column (or columns) in the child table are set to NULL.
• RESTRICT: if a row from the parent table has a matching row in the child table, MySQL rejects deleting or updating rows in the parent table.
• NO ACTION: is the same as RESTRICT.
• SET DEFAULT: is recognized by the MySQL parser. However, this action is rejected by both InnoDB and NDB tables.

In fact, MySQL fully supports three actions: RESTRICT, CASCADE and SET NULL.

If you don’t specify the ON DELETE and ON UPDATE clause, the default action is RESTRICT.

MySQL FOREIGN KEY examples

Let’s create a new database called fkdemo for the demonstration.

RESTRICT & NO ACTION actions

Inside the fkdemo database, create two tables categories and products:

The categoryId in the products table is the foreign key column that refers to the categoryId column in the categories table.

Because we don’t specify any ON UPDATE and ON DELETE clauses, the default action is RESTRICT for both update and delete operation.

The following steps illustrate the RESTRICT action.

1) Insert two rows into the categories table:

2) Select data from the categories table:

3) Insert a new row into the products table:

It works because the categoryId 1 exists in the categories table.

4) Attempt to insert a new row into the products table with a categoryId value does not exist in the categories table:

MySQL issued the following error:

5) Update the value in the categoryId column in the categories table to 100:

MySQL issued this error:

Because of the RESTRICT option, you cannot delete or update categoryId 1 since it is referenced by the productId1 in the products table.

CASCADE action

These steps illustrate how ON UPDATE CASCADE and ON DELETE CASCADE actions work.

1) Drop the products table:

2) Create the products table with the ON UPDATE CASCADE and ON DELETE CASCADE options for the foreign key:

3) Insert four rows into the products table:

4) Select data from the products table:

5) Update categoryId 1 to 100 in the categories table:

6) Verify the update:

7) Get data from the products table:

As you can see, two rows with value 1 in the categoryId column of the products table were automatically updated to 100 because of the ON UPDATE CASCADE action.

8) Delete categoryId 2 from the categories table:

9) Verify the deletion:

10) Check the products table:

All products with categoryId 2 from the products table were automatically deleted because of the ON DELETE CASCADE action.

SET NULL action

These steps illustrate how the ON UPDATE SET NULL and ON DELETE SET NULL actions work.

1) Drop both categories and products tables:

2) Create the categories and products tables:

The foreign key in the products table changed to ON UPDATE SET NULL and ON DELETE SET NULL options.

3) Insert rows into the categories table:

4) Insert rows into the products table:

5) Update categoryId from 1 to 100 in the categories table:

6) Verify the update:

7) Select data from the products table:

The rows with the categoryId 1 in the products table were automatically set to NULL due to the ON UPDATE SET NULL action.

8) Delete the categoryId 2 from the categories table:

Why Doesn't Rails Generate A Foreign Key In Excel

9) Check the products table:

The values in the categoryId column of the rows with categoryId 2 in the products table were automatically set to NULL due to the ON DELETE SET NULL action.

Drop MySQL foreign key constraints

To drop a foreign key constraint, you use the ALTER TABLE statement:

In this syntax:

• First, specify the name of the table from which you want to drop the foreign key after the ALTER TABLE keywords.
• Second, specify the constraint name after the DROP FOREIGN KEY keywords.

Notice that constraint_name is the name of the foreign key constraint specified when you created or added the foreign key constraint to the table.

To obtain the generated constraint name of a table, you use the SHOW CREATE TABLE statement:

Why Doesn't Rails Generate A Foreign Key In Spanish

For example, to see the foreign keys of the products table, you use the following statement:

The following is the output of the statement:

As you can see clearly from the output, the table products table has one foreign key constraint: fk_category

And this statement drops the foreign key constraint of the products table:

To ensure that the foreign key constraint has been dropped, you can view the structure of the products table:

Why Doesn't Rails Generate A Foreign Key Examples

Disabling foreign key checks

Sometimes, it is very useful to disable foreign key checks e.g., when you import data from a CSV file into a table. If you don’t disable foreign key checks, you have to load data into a proper order i.e., you have to load data into parent tables first and then child tables, which can be tedious. However, if you disable the foreign key checks, you can load data into tables in any order.

To disable foreign key checks, you use the following statement:

And you can enable it by using the following statement:

In this tutorial, you have learned about the MySQL foreign key and how to create a foreign key constraint with various reference options.