Ember .save() is not a function

I am using Ember with Firebase. I am trying to update two user's friends' lists when an action is executed. The way the model's work is that is each user belongs to a friendsList. Each friendsList has many friends (which are users). However when I run this code, I get the error:

currentFriendsList.save is not a function

Any ideas why this is happening?

Here is the code:

var userId = this.controllerFor('teams/new').get('userId');
      let currentUser = this.get('user').get('currentUser');
      let myUser = this.get('store').peekRecord('user', currentUser.id);
      let actorName = myUser.get('name');
      if (userId != null){
      let selectedUser = this.get('store').peekRecord('user', userId);
      let selectedFriendsList = selectedUser.get('friendsList');
      let currentFriendsList = myUser.get('friendsList');
        selectedFriendsList.get('friends').pushObject(myUser);
        currentFriendsList.get('friends').pushObject(selectedUser);
        currentFriendsList.save().then(() => 
        selectedFriendsList.save());

Here is the friendslist model:

user: DS.belongsTo('user', { inverse: 'friendsList' }),
    friends: DS.hasMany('user')

Here is the applicable part of the user model:

friendsList: DS.belongsTo('friendslist', { inverse: 'user' })