Rails Generate Model Foreign Key References Terminal
- Rails Generate Model Foreign Key References Terminal 4
- Primary Key
- Rails Generate Model Foreign Key References Terminal Code
- Rails Generate Model Foreign Key References Terminal 2
SQL FOREIGN KEY Constraint
A FOREIGN KEY is a key used to link two tables together.
If you run a generator such as: rails generate model accounts supplier:references The resulting migration will now add the corresponding foreign key constraint unless the reference was specified to be polymorphic. Generators add foreign keys on references #17759. I agree a foreign key option on references would be nice but we can.
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.
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Look at the following two tables:
'Persons' table:
| PersonID | LastName | FirstName | Age |
|---|---|---|---|
| 1 | Hansen | Ola | 30 |
| 2 | Svendson | Tove | 23 |
| 3 | Pettersen | Kari | 20 |
'Orders' table:
| OrderID | OrderNumber | PersonID |
|---|---|---|
| 1 | 77895 | 3 |
| 2 | 44678 | 3 |
| 3 | 22456 | 2 |
| 4 | 24562 | 1 |
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:
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
SQL Server / Oracle / MS Access:
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);
Rails Generate Model Foreign Key References Terminal 4
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:
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
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:
Primary Key
MySQL / SQL Server / Oracle / MS Access:
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
Rails Generate Model Foreign Key References Terminal Code
MySQL / SQL Server / Oracle / MS Access:
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:
Rails Generate Model Foreign Key References Terminal 2
MySQL:
DROP FOREIGN KEY FK_PersonOrder;
SQL Server / Oracle / MS Access:
DROP CONSTRAINT FK_PersonOrder;