How to Implement Conditional Foreign Key Constraints in MySQL for Polymorphic Associations?

Conditional foreign key constraints in MySQL: solving polymorphic association problems

Foreign key constraints enforce data integrity by ensuring that values ​​in one table refer to corresponding values ​​in another related table. However, in some cases, a table may require a foreign key that references a different table based on conditions. This is called a Conditional Foreign Key Constraint.

In this case, the challenge is to create a Comments table that can store comments for various parts of the application. For example, comments on blog posts and user images. Ideally, foreign key constraints on Comments in the foreign_id table should only be applied when model is set to 'blogpost'.

Solution: Polymorphic Association

Unfortunately, MySQL does not directly support conditional foreign key constraints. Instead, the solution involves using a design pattern called Polymorphic Association. This schema introduces an intermediate table named Commentable.

CREATE TABLE Commentable (
  id SERIAL PRIMARY KEY
);

CREATE TABLE Comments (
  comment_id SERIAL PRIMARY KEY,
  foreign_id INT NOT NULL,
  ...
  FOREIGN KEY (foreign_id) REFERENCES Commentable(id)
);

Each content type (e.g., BlogPost, UserPicture) becomes a subtype of Commentable and references it via a foreign key.

CREATE TABLE BlogPosts (
  blogpost_id INT PRIMARY KEY, -- 非自增
  ...
  FOREIGN KEY (blogpost_id) REFERENCES Commentable(id)
);

CREATE TABLE UserPictures (
  userpicture_id INT PRIMARY KEY, -- 非自增
  ...
  FOREIGN KEY (userpicture_id) REFERENCES Commentable(id)
);

Before adding data to these subtype tables, new rows must be inserted into the Commentable table to generate unique IDs. This generated ID is then used as a foreign key in the subtype table.

By using this design pattern, you can enforce referential integrity constraints, ensuring that comments on blog posts only reference blog posts, and comments on user pictures only reference user pictures.

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