Firebase members rules security

I have a data storaged in firestone like this:

The members field storage two others fields, admin and viewers, and i need to restrict the read function only to the viewer member, in the rules of my firestone database i did this

service cloud.firestore {
  match /databases/{database}/documents {
    match /constructions/{construction=**} {    	
      function canRead() {
      	return resource.data.members.viewer[request.auth.uid]
      }      
      allow read: if canRead()
      allow write: if request.auth != null   
    }
  }
}