能否在Cloud Firestore规则中使用模板字符串变量实现动态字段校验？

Great question! You're spot on that the $(something) template syntax only works when defining document paths in Firestore Security Rules—it won't let you dynamically reference field names using that syntax. But don't worry, there's a simple workaround using bracket notation that does exactly what you need.

The Solution #

Instead of trying to use the template syntax for field access, use square brackets to dynamically reference the field key using the user's UID:

allow update: if request.auth != null && 
               request.auth.uid == resource.data.players[request.auth.uid].id;

How It Works #

In Firestore Rules, when you need to access an object's property using a variable (like request.auth.uid here), bracket notation [] replaces the dot notation. This lets you:

• Check that the user is authenticated (request.auth != null—always a good practice to avoid null errors)
• Dynamically target the specific entry in the players object using the user's UID as the key
• Validate that the id field inside that entry matches the user's authenticated UID

Extra Safety Check #

To avoid errors if the user's UID isn't present in the players object, add an existence check first:

allow update: if request.auth != null && 
               request.auth.uid in resource.data.players &&
               request.auth.uid == resource.data.players[request.auth.uid].id;

This ensures the rule only proceeds to validate the id field if the user's entry actually exists in the players object, preventing unexpected rule failures.

内容的提问来源于stack exchange，提问作者nialna2