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I followed this tutorial for implementing friend requests https://dankim.io/mutual-friendship-rails/ and one of the first steps is to create the friendship model with a friend reference although there is no friend table. This does work locally, I followed the whole tutorial and creating and accepting requests all work and I haven't had any problems until I just tried to push to heroku. The error I'm getting is PG::UndefinedTable: ERROR: relation "friends" does not exist.

schema:

ActiveRecord::Schema.define(version: 20171217055455) do
  # These are extensions that must be enabled in order to support this database
  enable_extension "plpgsql"
  create_table "comments", force: :cascade do |t|
    t.bigint "post_id"
    t.string "name"
    t.string "body"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["post_id"], name: "index_comments_on_post_id"
  create_table "friend_requests", force: :cascade do |t|
    t.bigint "user_id"
    t.bigint "friend_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["friend_id"], name: "index_friend_requests_on_friend_id"
    t.index ["user_id"], name: "index_friend_requests_on_user_id"
  create_table "friendships", force: :cascade do |t|
    t.bigint "user_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.bigint "friend_id"
    t.index ["friend_id"], name: "index_friendships_on_friend_id"
    t.index ["user_id"], name: "index_friendships_on_user_id"
  create_table "likes", force: :cascade do |t|
    t.bigint "user_id"
    t.bigint "post_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["post_id"], name: "index_likes_on_post_id"
    t.index ["user_id"], name: "index_likes_on_user_id"
  create_table "posts", force: :cascade do |t|
    t.string "body"
    t.bigint "user_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["user_id"], name: "index_posts_on_user_id"
  create_table "users", force: :cascade do |t|
    t.string "email", default: "", null: false
    t.string "encrypted_password", default: "", null: false
    t.string "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer "sign_in_count", default: 0, null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.inet "current_sign_in_ip"
    t.inet "last_sign_in_ip"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.string "name"
    t.index ["email"], name: "index_users_on_email", unique: true
    t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
  create_table "votes", force: :cascade do |t|
    t.string "votable_type"
    t.bigint "votable_id"
    t.string "voter_type"
    t.bigint "voter_id"
    t.boolean "vote_flag"
    t.string "vote_scope"
    t.integer "vote_weight"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["votable_id", "votable_type", "vote_scope"], name: "index_votes_on_votable_id_and_votable_type_and_vote_scope"
    t.index ["votable_type", "votable_id"], name: "index_votes_on_votable_type_and_votable_id"
    t.index ["voter_id", "voter_type", "vote_scope"], name: "index_votes_on_voter_id_and_voter_type_and_vote_scope"
    t.index ["voter_type", "voter_id"], name: "index_votes_on_voter_type_and_voter_id"
  add_foreign_key "comments", "posts"
  add_foreign_key "friend_requests", "users"
  add_foreign_key "friendships", "users"
  add_foreign_key "likes", "posts"
  add_foreign_key "likes", "users"
  add_foreign_key "posts", "users"
After dropping my database locally and trying to migrate I ended up with the same error. This lead me to look deeper into my migrations and I found my error.
Migration causing error:
class CreateFriendships < ActiveRecord::Migration[5.1]
  def change
    create_table :friendships do |t|
      t.references :user, foreign_key: true
      t.references :friend, foreign_key: true 
      t.timestamps
Since there is no Friend model foreign_key: true caused the error. I think this is because foreign_key: true adds a belongs_to :friendships in what would have been the Friend model.  
class CreateFriendships < ActiveRecord::Migration[5.1]
  def change
    create_table :friendships do |t|
      t.references :user, foreign_key: true
      t.references :friend
      t.timestamps
For the friend reference you need to specify the column name of the foreign key.
class CreateFriendships < ActiveRecord::Migration[5.2]
  def change
    create_table :friendships do |t|
      t.references :user, foreign_key: true
      t.references :friend
      t.timestamps
    add_foreign_key :friendships, :users, column: :friend_id
        
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