finally corrected that model thing.

app/models/group.rb

@@ -6,7 +6,13 @@ class Group < ActiveRecord::Base
     self.name = self.display_name.downcase.sub(/\s+/, '-')
   end
 
+  alias members group_members
+
   def add_member(user)
     GroupMember.create! group: self, user: user
   end
+
+  def remove_member(user)
+    GroupMember.where(group: self, user: user).first!.destroy
+  end
 end

app/models/group_member.rb

@@ -1,4 +1,4 @@
 class GroupMember < ActiveRecord::Base
-  has_one :user
-  has_one :group
+  belongs_to :user
+  belongs_to :group
 end

app/models/user.rb

@@ -26,7 +26,7 @@ class User < ActiveRecord::Base
   has_many :moderation_comments, dependent: :destroy
   has_many :moderation_votes, dependent: :destroy
   has_many :groups, dependent: :destroy
-  has_many :group_memberships, class_name: "GroupMember", dependent: :destroy
+  has_many :group_memberships, class_name: "GroupMember", foreign_key: 'user_id', dependent: :destroy
 
   SCREEN_NAME_REGEX = /\A[a-zA-Z0-9_]{1,16}\z/
   WEBSITE_REGEX = /https?:\/\/([A-Za-z.\-]+)\/?(?:.*)/i