Use integer instead of bigint in DB migrations. And hope that this will turn out suitable for post IDs.

db/migrate/20131208115327_create_reposts.rb

@@ -2,8 +2,8 @@ class CreateReposts < ActiveRecord::Migration
   def change
     create_table :reposts do |t|
       t.string :mentioned
-      t.bigint :postid
-      t.bigint :repostid
+      t.integer :postid
+      t.integer :repostid
 
       t.timestamps
     end

db/schema.rb

@@ -0,0 +1,29 @@
+# encoding: UTF-8
+# This file is auto-generated from the current state of the database. Instead
+# of editing this file, please use the migrations feature of Active Record to
+# incrementally modify your database, and then regenerate this schema definition.
+#
+# Note that this schema.rb definition is the authoritative source for your
+# database schema. If you need to create the application database on another
+# system, you should be using db:schema:load, not running all the migrations
+# from scratch. The latter is a flawed and unsustainable approach (the more migrations
+# you'll amass, the slower it'll run and the greater likelihood for issues).
+#
+# It's strongly recommended that you check this file into your version control system.
+
+ActiveRecord::Schema.define(version: 20131208115327) do
+
+  # These are extensions that must be enabled in order to support this database
+  enable_extension "plpgsql"
+
+  create_table "reposts", force: true do |t|
+    t.string   "mentioned"
+    t.integer  "postid"
+    t.integer  "repostid"
+    t.datetime "created_at"
+    t.datetime "updated_at"
+  end
+
+  add_index "reposts", ["repostid"], name: "index_reposts_on_repostid", using: :btree
+
+end