Firebase limit update to a custom field

In Firebase, here is my Firestore :

I am using this function to Update Like :

 const docRef = await doc(db, "databasetest", "explaura");
           const Path = `${_Selected.Name}.Like`;
            await updateDoc(docRef, {
                [Path]: increment(1)
            });

So _Selected.Name is Dynamic (Reference as DynamicName on the screen Shot).

How can I create a Firebase rules to allow ONLY like Update ? Found solution with request.resource.data.keys().hasAll but it's not Working. I can Update all child of DynamicName or none, but not Only "Like"

Can you help me please ?

I also Tried something like that :

match /databasetest/explaura/{id}{
      allow read: if true;
      allow update: if (request.resource.data.diff(resource.data).affectedKeys().hasOnly(['Like']));
    }

All methods of rules.MapDiff return a rules.Set which does not contain an index operator, like rules.List does. This prevents using diff for your purpose (I have created a ticket to Firebase like 2 years ago to have this added to the API but haven't got any news since, feel free to create one yourself).

Fortunatly there is a deprecated property request.writeFields that you can use and that fits your need. It contains all the fields that have been changed (updated, created, deleted):

match /databasetest/explaura/{id}{
  allow read: if true;
  allow update: if request.writeFields.size() == 1  // Only one field is updated
    && validate_update();

  function validate_update() {
    let fields = request.writeFields[0].split('\\.');  // In your case request.writeFields should be equal to ['DynamicName.like']
    return fields.size() == 2    // ['DynamicName','like']
      && fields[0].size() <= 30  // Some condition on DynamicName
      && fields[1] == 'like'     // Only field updated is 'like'
      && request.resource.data.get(fields, 0) = 1 + resource.data.get(fields, 0) // We only accept increment, modify as you wish
  }
}

Here I use the version of rules.Map.get accepting an array as parameter to retrieve nested fields.

Notes:

1. request.writeFields is deprecated but still working, use at your own risk.
2. My guess is that it has been deprecated because its behavior can be a bit weird in some edge cases, especially for nested fields.
3. Still, I find it crazy to mark something as deprecated when there is no new solution offering the same functionality (heads up to the firebaser here...)