Generate New Table With Foreign Key Rails
SQL FOREIGN KEY Constraint
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A FOREIGN KEY is a key used to link two tables together.
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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.
By convention, Rails guesses that the column in the join table used to hold the foreign key pointing to the other model is the name of that model with the suffix id added. The:associationforeignkey option lets you set the name of the foreign key directly. Oct 23, 2009 CONSTRAINT `fkusersroles` FOREIGN KEY (`roleid`) REFERENCES `role`( `id`) ON DELETE CASCADE) ENGINE = innodb; Next, let us create the table for Story, we will call it the ‘tales’ table, we will also add a foreign key reference to the users table in it. Here is the query for creating the table. CREATE TABLE `tales`. Adding foreign key to a rails model. Index on any foreign key you create or generate. You can either use t.belongsto:, index: true in the create table. 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. Sep 24, 2019 Rails provides addforeignkey to add foreign key constraint for a column on a table. It also provides removeforeignkey to remove the foreign key constraint. Before Rails 6, addforeignkey and removeforeignkey were not supported for SQLite3.
Look at the following two tables:
'Persons' table:
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| 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:
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MySQL:
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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)
);
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:
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:
Generate New Table With Foreign Key Rails 2017
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:
MySQL:
DROP FOREIGN KEY FK_PersonOrder;
SQL Server / Oracle / MS Access:
Generate New Table With Foreign Key Rails For Windows
DROP CONSTRAINT FK_PersonOrder;