How Can I Implement Polymorphic Associations in MySQL to Handle Conditional Foreign Keys?

Conditional foreign keys in MySQL

In database design, it is often necessary to enforce foreign key constraints to ensure data consistency. However, in scenarios involving polymorphism, foreign keys can reference rows in multiple related tables, and traditional foreign key constraints are insufficient.

In the example provided, the comments table contains a foreign_id field, which can reference different tables (blogposts, picture, etc.) based on the model field. To do this, a conditional foreign key constraint is required.

MySQL itself does not support conditional foreign key constraints. Declaring a foreign key that references a different table based on a condition would violate relational database design principles.

A more efficient approach, called "polymorphic association", involves creating a "supertable" that serves as a single reference point for all related tables. In this case, the following structure can be implemented:

<code class="language-sql">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)
);</code>

Each content type (blog post, user image, etc.) becomes a subtype of the Commentable supertable:

<code class="language-sql">CREATE TABLE BlogPosts (
  blogpost_id INT PRIMARY KEY, -- 非自动生成
  ...
  FOREIGN KEY (blogpost_id) REFERENCES Commentable(id)
);

CREATE TABLE UserPictures (
  userpicture_id INT PRIMARY KEY, -- 非自动生成
  ...
  FOREIGN KEY (userpicture_id) REFERENCES Commentable(id)
);</code>

Inserting data into a specific content table requires first inserting a new row into the Commentable table to obtain the pseudo primary key id. This id can then be used for a specific content table.

This approach allows referential integrity while maintaining the flexibility required for polymorphic associations.

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