- Rails Generate Model Foreign Key References Terminal 4
- Rails Generate Model Foreign Key References Terminal 4
- Primary Key
- Rails Generate Model Foreign Key References Terminal 1
- Foreign Key In Mysql
- Rails Generate Model Foreign Key References Terminal 1
SQL FOREIGN KEY Constraint
A FOREIGN KEY is a key used to link two tables together.
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.
Ruby on Rails. Contribute to rails/rails development by creating an account on GitHub. There’s one difference in this generated file: using references does actually result in another line of code being added to the model, a call to belongsto.This indicates that a Dish belongs to a Restaurant: that is, it has a foreign key pointing to it. 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. 使い方 refernces型は外部キーを追加する時に使います 今回はtweetsモデルにuseridを追加します。(user hasmany tweet) class CreateTweets < ActiveRec.
Look at the following two tables:
'Persons' table:
PersonID | LastName | FirstName | Age |
---|---|---|---|
1 | Hansen | Ola | 30 |
2 | Svendson | Tove | 23 |
3 | Pettersen | Kari | 20 |
Rails Generate Model Foreign Key References Terminal 4
'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:
CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
SQL Server / Oracle / MS Access:
CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);
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:
CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
Rails Generate Model Foreign Key References Terminal 4
![Key Key](/uploads/1/2/5/7/125756904/102167085.gif)
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:
ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
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:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
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:
Primary Key
MySQL:
Rails Generate Model Foreign Key References Terminal 1
ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;
DROP FOREIGN KEY FK_PersonOrder;
SQL Server / Oracle / MS Access:
Foreign Key In Mysql
ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;
DROP CONSTRAINT FK_PersonOrder;