added Group model

app/models/group.rb

@@ -0,0 +1,4 @@
+class Group < ActiveRecord::Base
+  belongs_to :user
+  has_many :users
+end

app/models/user.rb

@@ -25,6 +25,8 @@ class User < ActiveRecord::Base
   has_many :reports, dependent: :destroy
   has_many :moderation_comments, dependent: :destroy
   has_many :moderation_votes, dependent: :destroy
+  has_many :groups, dependent: :destroy
+  belongs_to :group, foreign_key: :target_id
 
   SCREEN_NAME_REGEX = /\A[a-zA-Z0-9_]{1,16}\z/
   WEBSITE_REGEX = /https?:\/\/([A-Za-z.\-]+)\/?(?:.*)/i

db/migrate/20150108121010_create_groups.rb

@@ -0,0 +1,17 @@
+class CreateGroups < ActiveRecord::Migration
+  def change
+    create_table :groups do |t|
+      t.integer :user_id, null: false
+      t.integer :target_id, null: false
+      t.string :name
+      t.string :display_name
+
+      t.timestamps
+    end
+
+    add_index :groups, :user_id
+    add_index :groups, :target_id
+    add_index :groups, :name
+    add_index :groups, [:user_id, :name], unique: true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20150103200732) do
+ActiveRecord::Schema.define(version: 20150108121010) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -38,6 +38,20 @@ ActiveRecord::Schema.define(version: 20150103200732) do
 
   add_index "comments", ["user_id", "created_at"], name: "index_comments_on_user_id_and_created_at", using: :btree
 
+  create_table "groups", force: true do |t|
+    t.integer  "user_id",      null: false
+    t.integer  "target_id",    null: false
+    t.string   "name"
+    t.string   "display_name"
+    t.datetime "created_at"
+    t.datetime "updated_at"
+  end
+
+  add_index "groups", ["name"], name: "index_groups_on_name", using: :btree
+  add_index "groups", ["target_id"], name: "index_groups_on_target_id", using: :btree
+  add_index "groups", ["user_id", "name"], name: "index_groups_on_user_id_and_name", unique: true, using: :btree
+  add_index "groups", ["user_id"], name: "index_groups_on_user_id", using: :btree
+
   create_table "inboxes", force: true do |t|
     t.integer  "user_id"
     t.integer  "question_id"