Update many-to-many association in rails (User-Team relationship aka Membership) - ruby-on-rails

Question

I just created a create feature for adding a team together with adding the members to a said team.
The form contains the following:
Name of the team.
Team's department.
Leader (value is the user id, but displayed as full name)
Members (values are user ids, but displayed also as full names) //I used a special select menu called select2.
Here are the models. I'll only show the associations and some methods related to my problem.
user.rb
class User < ActiveRecord::Base
has_many :memberships
has_many :teams, through: :memberships
accepts_nested_attributes_for :memberships, :teams
end
team.rb
class Team < ActiveRecord::Base
has_many :memberships
has_many :teams, through: :memberships
accepts_nested_attributes_for :memberships, :users
def build_membership(user_ids)
unless user_ids.blank?
user_ids.each do |id|
self.users << User.find_by_id(id)
end
end
end
end
membership.rb
class Membership < ActiveRecord::Base
belongs_to :team
belongs_to :user
#Note that leader and members are both users
end
Here is the controller, with the create and update methods.
class TeamsController < ApplicationController
def create
#team = Team.new(team_params)
#team.build_membership(build_members_array(members_params))
if #team.save
flash.now[:success] = 'Team was successfully created.'
redirect_to #team
else
flash.now[:notice] = #team.errors.full_messages
render "new"
end
end
def update
# TO-DO: update leader and members (i.e. add or remove member)
if #team.update(team_params)
flash.now[:success] = "Team was successfully updated."
redirect_to #team
else
flash.now[:notice] = #team.errors.full_messages
render "edit"
end
end
private
def team_params
params.require(:team).permit(:name,:department)
end
def members_params
params.require(:team).permit(:leader, members:[])
end
def build_user_ids_array(members)
#put ids of leader and members in an array
end
end
It seems that only the name and department attributes are only updated while the leader and the members are not. Should I create my own method again for updating the roster of the team or do something else in mind?

Answer 1

#team.build_membership would be if you are passing in the params to create a new user, but you are using existing User's so this wouldn't apply. You will want to instead fetch all of the User's by their id and then add that the the Team. You also need to permit the id in the members_attributes in your member_params method.
Something like:
def create
#team = Team.new(team_params)
#team.memberships << users_from_params
if #team.save
flash.now[:success] = 'Team was successfully created.'
redirect_to #team
else
flash.now[:notice] = #team.errors.full_messages
render "new"
end
end
def members_params
params.require(:team).permit(:leader, members_attributes: [:id])
end
def users_from_params
#This members_params[:members_attributes].values should be an array of `id`s
User.find(members_params[:members_attributes].values)
end