type Zoo {
  ...
  customAnimalsNames: [String]
  animals: [Animal]
  city: String!
}

type List {
  ...
  zoo: [Zoo]
  ...
}

type Animal {
  ...
  name: String!
}

import Animals from './collection';

export default (root, context, {userId}) => {
  ...
  return Animals.find({}).fetch();
}

list(_id: String){
   zoo: {
     animals
   }

}