verifying rails associations exist

rails_association_testing.rb

# belongs_to side contains the foreign key
class Comment < ActiveRecord::Base
  belongs_to :example
end

# has_one side does not contain the foreign key
class Example < ApplicationRecord
  belongs_to :user
  has_one :comment
  validates :text, :user, presence: true
end

# Below is creating and testing via the rails console:

[email protected]: rails-api-bdd (training-017)* $ rails g migration addExampleIdToComment example:references
Running via Spring preloader in process 66200
Expected string default value for '--serializer'; got true (boolean)
      invoke  active_record
      create    db/migrate/20170512151033_add_example_id_to_comment.rb
[email protected]: rails-api-bdd (training-017)* $ vim .
[email protected]: rails-api-bdd (training-017)* $ rails db:migrate
== 20170512151033 AddExampleIdToComment: migrating ============================
-- add_reference(:comments, :example, {:foreign_key=>true})
   -> 0.0877s
== 20170512151033 AddExampleIdToComment: migrated (0.0878s) ===================

[email protected]: rails-api-bdd (training-017)* $ rails c
Running via Spring preloader in process 66670
Loading development environment (Rails 5.0.1)
[1] pry(main)> comment = Comment.new
=> #<Comment:0x007fcf0d4513c8 id: nil, body: nil, created_at: nil, updated_at: nil, article_id: nil, example_id: nil>
[2] pry(main)> comment.example.build
NoMethodError: undefined method `build' for nil:NilClass
from (pry):2:in `<main>'
[3] pry(main)> comment.build_example
=> #<Example:0x007fcf0a9f6e70 id: nil, text: nil, user_id: nil, created_at: nil, updated_at: nil>
[4] pry(main)> comment.example.nil?
=> false
[5] pry(main)> comment.example
=> #<Example:0x007fcf0a9f6e70 id: nil, text: nil, user_id: nil, created_at: nil, updated_at: nil>
[6] pry(main)> comment.save
   (0.2ms)  BEGIN
  SQL (18.7ms)  INSERT INTO "comments" ("created_at", "updated_at") VALUES ($1, $2) RETURNING "id"  [["created_at", 2017-05-12 15:21:00 UTC], ["updated_at", 2017-05-12 15:21:00 UTC]]
   (0.7ms)  COMMIT
=> true
[7] pry(main)> comment
=> #<Comment:0x007fcf0d4513c8
 id: 1,
 body: nil,
 created_at: Fri, 12 May 2017 15:21:00 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:21:00 UTC +00:00,
 article_id: nil,
 example_id: nil>
[8] pry(main)> comment.example
=> #<Example:0x007fcf0a9f6e70 id: nil, text: nil, user_id: nil, created_at: nil, updated_at: nil>
[9] pry(main)> comment.example.valid?
=> false
[10] pry(main)> comment.example.errors
=> #<ActiveModel::Errors:0x007fcf0cb9b2b0
 @base=#<Example:0x007fcf0a9f6e70 id: nil, text: nil, user_id: nil, created_at: nil, updated_at: nil>,
 @details={:user=>[{:error=>:blank}, {:error=>:blank}], :text=>[{:error=>:blank}]},
 @messages={:user=>["must exist", "can't be blank"], :text=>["can't be blank"]}>
[11] pry(main)> comment.example.user
=> nil
[12] pry(main)> comment.example.user = User.last
  User Load (0.6ms)  SELECT  "users".* FROM "users" ORDER BY "users"."id" DESC LIMIT $1  [["LIMIT", 1]]
=> nil
[13] pry(main)> User.create
   (0.2ms)  BEGIN
  User Exists (0.5ms)  SELECT  1 AS one FROM "users" WHERE "users"."email" IS NULL LIMIT $1  [["LIMIT", 1]]
   (5.4ms)  ROLLBACK
=> #<User:0x007fcf0d002ad8 id: nil, email: nil, token: nil, password_digest: nil, created_at: nil, updated_at: nil>
[14] pry(main)> User.create(email: 'd@k')
   (0.2ms)  BEGIN
  User Exists (0.7ms)  SELECT  1 AS one FROM "users" WHERE "users"."email" = $1 LIMIT $2  [["email", "d@k"], ["LIMIT", 1]]
   (0.2ms)  ROLLBACK
=> #<User:0x007fcf09f138a0 id: nil, email: "d@k", token: nil, password_digest: nil, created_at: nil, updated_at: nil>
[15] pry(main)> User.create(email: 'd@k', password: 'q')
   (0.2ms)  BEGIN
  User Exists (0.3ms)  SELECT  1 AS one FROM "users" WHERE "users"."email" = $1 LIMIT $2  [["email", "d@k"], ["LIMIT", 1]]
  SQL (8.7ms)  INSERT INTO "users" ("email", "token", "password_digest", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["email", "d@k"], ["token", "939a704cb70210a3386d013336c109b5"], ["password_digest", "$2a$10$h8nFhz43zbBskZz3n1HSrujOB9N64hOyuvdkghouegij0DkSP1Cvy"], ["created_at", 2017-05-12 15:21:40 UTC], ["updated_at", 2017-05-12 15:21:40 UTC]]
   (0.5ms)  COMMIT
=> #<User:0x007fcf0c8e3f18
 id: 1,
 email: "d@k",
 token: "939a704cb70210a3386d013336c109b5",
 password_digest: "$2a$10$h8nFhz43zbBskZz3n1HSrujOB9N64hOyuvdkghouegij0DkSP1Cvy",
 created_at: Fri, 12 May 2017 15:21:40 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:21:40 UTC +00:00>
[16] pry(main)> comment.example.user = User.last
  User Load (0.4ms)  SELECT  "users".* FROM "users" ORDER BY "users"."id" DESC LIMIT $1  [["LIMIT", 1]]
=> #<User:0x007fcf09e6ae80
 id: 1,
 email: "d@k",
 token: "939a704cb70210a3386d013336c109b5",
 password_digest: "$2a$10$h8nFhz43zbBskZz3n1HSrujOB9N64hOyuvdkghouegij0DkSP1Cvy",
 created_at: Fri, 12 May 2017 15:21:40 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:21:40 UTC +00:00>
[17] pry(main)> comment.example.valid?
=> false
[18] pry(main)> comment.example.errors
=> #<ActiveModel::Errors:0x007fcf0cb9b2b0
 @base=#<Example:0x007fcf0a9f6e70 id: nil, text: nil, user_id: 1, created_at: nil, updated_at: nil>,
 @details={:text=>[{:error=>:blank}]},
 @messages={:text=>["can't be blank"]}>
[19] pry(main)> comment.example.text = "blah"
=> "blah"
[20] pry(main)> comment.example.valid?
=> true
[21] pry(main)> comment.example.save
   (0.2ms)  BEGIN
  SQL (9.7ms)  INSERT INTO "examples" ("text", "user_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["text", "blah"], ["user_id", 1], ["created_at", 2017-05-12 15:22:03 UTC], ["updated_at", 2017-05-12 15:22:03 UTC]]
  Example Load (0.3ms)  SELECT  "examples".* FROM "examples" WHERE "examples"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  SQL (0.7ms)  UPDATE "comments" SET "updated_at" = $1, "example_id" = $2 WHERE "comments"."id" = $3  [["updated_at", 2017-05-12 15:22:03 UTC], ["example_id", 1], ["id", 1]]
   (5.7ms)  COMMIT
=> true
[22] pry(main)> comment.exapmle
NoMethodError: undefined method `exapmle' for #<Comment:0x007fcf0d4513c8>
Did you mean?  example
from /Users/dkirschner/.rvm/gems/ruby-2.3.1/gems/activemodel-5.0.1/lib/active_model/attribute_methods.rb:433:in `method_missing'
[23] pry(main)> comment.example
=> #<Example:0x007fcf09e40180
 id: 1,
 text: "blah",
 user_id: 1,
 created_at: Fri, 12 May 2017 15:22:03 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:22:03 UTC +00:00>
[24] pry(main)> comment.example.nil?
=> false
[25] pry(main)> Example.last
  Example Load (0.3ms)  SELECT  "examples".* FROM "examples" ORDER BY "examples"."id" DESC LIMIT $1  [["LIMIT", 1]]
=> #<Example:0x007fcf0a5b07f8
 id: 1,
 text: "blah",
 user_id: 1,
 created_at: Fri, 12 May 2017 15:22:03 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:22:03 UTC +00:00>
[26] pry(main)> Example.last.comment
  Example Load (0.4ms)  SELECT  "examples".* FROM "examples" ORDER BY "examples"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."example_id" = $1 LIMIT $2  [["example_id", 1], ["LIMIT", 1]]
=> #<Comment:0x007fcf09cd2398
 id: 1,
 body: nil,
 created_at: Fri, 12 May 2017 15:21:00 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:22:03 UTC +00:00,
 article_id: nil,
 example_id: 1>
[27] pry(main)> Example.last.comments
  Example Load (0.5ms)  SELECT  "examples".* FROM "examples" ORDER BY "examples"."id" DESC LIMIT $1  [["LIMIT", 1]]
NoMethodError: undefined method `comments' for #<Example:0x007fcf09c7b7f0>
Did you mean?  comment
               comment=
from /Users/dkirschner/.rvm/gems/ruby-2.3.1/gems/activemodel-5.0.1/lib/active_model/attribute_methods.rb:433:in `method_missing'
[28] pry(main)> Example.last.comment
  Example Load (0.4ms)  SELECT  "examples".* FROM "examples" ORDER BY "examples"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Comment Load (0.4ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."example_id" = $1 LIMIT $2  [["example_id", 1], ["LIMIT", 1]]
=> #<Comment:0x007fcf0a4da978
 id: 1,
 body: nil,
 created_at: Fri, 12 May 2017 15:21:00 UTC +00:00,
 updated_at: Fri, 12 May 2017 15:22:03 UTC +00:00,
 article_id: nil,
 example_id: 1>
[29] pry(main)> Example.last.comment.nil?
  Example Load (0.3ms)  SELECT  "examples".* FROM "examples" ORDER BY "examples"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Comment Load (0.3ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."example_id" = $1 LIMIT $2  [["example_id", 1], ["LIMIT", 1]]
=> false
[30] pry(main)>