如何為REST API註冊自定義字段

要向WordPress的REST API添加自定義字段，需使用register_rest_field()或register_meta()。 1. 使用register_rest_field()處理非元數據的自定義數據，通過rest_api_init鉤子註冊字段，並定義get_callback、update_callback及權限控制；2. 使用register_meta()暴露存儲在post meta或user meta中的字段，只需設置show_in_rest為true；3. 測試新字段時訪問站點的REST API端點，確保代碼生效並檢查權限設置。兩種方法均需注意刷新規則和權限配置以確保字段正確顯示和訪問。

To add a custom field to the REST API in WordPress, you need to register it using a filter or function. The most common way is with register_rest_field() or register_meta() , depending on what kind of data you're working with and where it's stored.

Here's how to do it step by step.

Use register_rest_field() for Custom Data

If your custom field isn't already part of the post meta or user meta system — say, it's calculated or pulled from somewhere else — use register_rest_field() . This gives you full control over how and when the data is retrieved and updated.

You'll typically hook into rest_api_init so your field gets registered when the REST API starts up.

 add_action( 'rest_api_init', function () {
    register_rest_field( 'post', 'custom_data', array(
        'get_callback' => 'get_custom_data',
        'update_callback' => 'update_custom_data',
        'schema' => null,
    ));
});

• 'post' means this will apply to posts. You can also use 'user' , 'comment' , or any custom post type.
• 'custom_data' is the name of the new field that will appear in the API response.
• get_callback defines how the value is fetched.
• update_callback handles updates (optional if you don't want the field to be editable).

Example get callback:

 function get_custom_data( $object, $field_name, $request ) {
    return get_post_meta( $object['id'], '_custom_key', true );
}

Use register_meta() for Post or User Meta

If your custom field lives in the post meta or user meta table, register_meta() is usually better. It automatically adds the field to the REST API if configured properly.

For example, to expose a _custom_key meta field for posts:

 register_meta( 'post', '_custom_key', array(
    'type' => 'string',
    'description' => 'A custom text field',
    'single' => true,
    'show_in_rest' => true,
));

This approach is cleaner and avoids writing extra callbacks unless needed.

If you're registering for users instead of posts, just change 'post' to 'user' .

Note: If you're using Advanced Custom Fields (ACF), make sure to set 'show_in_rest' => true in your ACF field group settings for the fields you want exposed.

Test Your New Field in the REST API

Once you've added the code, check your site's REST API endpoint to see if the field shows up. For posts, go to:

 https://yoursite.com/wp-json/wp/v2/posts

Click on a specific post, and you should see your custom field there.

If not:

• Make sure the code is active (in your theme's functions.php or a plugin).
• Check for syntax errors.
• Flush rewrite rules by visiting Settings > Permalinks and clicking Save.

Permissions and Security

By default, some fields may only show up if you're logged in or have the right capabilities. If you want your custom field to be publicly accessible, make sure to set permissions correctly.

For register_rest_field() , you can add a permission callback:

 'permission_callback' => function () {
    return current_user_can( 'edit_posts' );
},

Or for public read access:

 'permission_callback' => '__return_true',

Use caution with write permissions — only allow updates if absolutely necessary and validate all input.

That's basically it. It's not complicated once you know which function to use, but easy to miss small details like flush rules or permission settings.

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