Why Insert SQL.

why_insert.rb

#
# id | name
#
class User < ActionRecord::Base
end

#
# id | name
#
class Group < ActionRecord::Base
  has_many :members, class_name: :GroupMember, dependent: :destroy
end

#
# id | group_id | user_id
#
class GroupMember < ActionRecord::Base
  belongs_to :group
  belongs_to :user
end

# not run insert sql
@group = Group.includes(:members).find(params[:id])
member_users = Hash[*@group.members.map{|member| [member.user_id, member]}.flatten]
@group_members = User.all.map do |user|
  if member_users.include?(user.id)
    member_users[user.id].user = user
    member_users[user.id]
  else
    GroupMember.new(group_id: @group.id, user: user)
  end
end

# run insert sql
@group = Group.includes(:members).find(params[:id])
member_users = Hash[*@group.members.map{|member| [member.user_id, member]}.flatten]
@group.members = User.all.map do |user|
  if member_users.include?(user.id)
    member_users[user.id].user = user
    member_users[user.id]
  else
    GroupMember.new(group_id: @group.id, user: user)
  end
end