One-to-Many Model Associations

In this demonstration, I will show how to set up and use one-to-many model class associations. We will continue to build upon the QuizMe project from the previous demos.

In particular, we will be updating our model design by adding a new one-to-many association between the Quiz and McQuestion model classes, as depicted in Figure 1.

In order to implement this association, there are a series of high-level tasks we will perform:

1. Add a foreign key (FK) column to our existing mc_questions table by creating a new database migration. The purpose of this FK column will be to reference Quiz records. The FK column is necessary to satisfy the Rails ORM for one-to-many associations.
2. Add declarations to the Quiz and McQuestion model classes to set up the one-to-many association. In particular, Quiz will get a has_many declaration, and McQuestion will get a belongs_to declaration.
3. Update the model test fixtures to incorporate association links.
4. Create new seed data that include association links.

1. Adding a Foreign Key Column to the mc_questions Database Table

In preparation for creating the model class association, we will perform the following steps to add a new foreign key column to the database table mc_questions. This FK column will reference Quiz records and satisfy the Rails ORM for setting up a one-to-many association between Quiz and McQuestion.

Generate a new (empty) database migration by running the following command:

rails g migration AddQuizFkColToMcQuestions

This will generate a migration file db/migrate/20191106225052_add_quiz_fk_col_to_mc_questions.rb (with the timestamp being consistent with the time when you ran the command). Inspect this file, and note that it contains a class AddQuizFkColToMcQuestions with a single empty method change.

Set up the migration to add a quiz_id FK column to the mc_questions table by inserting the following line in the change method:

add_reference :mc_questions, :quiz, foreign_key: true

Note that the first argument (:mc_questions) indicates the table to add the column to; the second argument (:quiz) indicates the column name, which Rails automatically translates into quiz_id because it’s an FK, and the third argument (foreign_key: true) indicates that it’s a FK column.

Update the database schema by running the migration with the following command:

rails db:migrate

Verify that the database schema was updated correctly by using pgAdmin to inspect the mc_questions database table. The table should have a new quiz_id column.

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2. Creating a One-to-Many Association between the Quiz and McQuestion Model Classes

Now that we have got the database tables set up, the other thing we need to do to set up the one-to-many association is to add has_many and belongs_to declarations to Quiz and McQuestion, respectively.

Add the following belongs_to declaration to the McQuestion model class, as follows:

belongs_to(
  :quiz, # McQuestion attribute of with datatype Quiz
  class_name: 'Quiz', # datatype of attribute
  foreign_key: 'quiz_id', # name of column containing FK
  inverse_of: :mc_questions # attribute on other side of association (array containing all McQuestion objects belonging to a quiz)
)

In the above belongs_to declaration, we explicitly set all the key options, class_name, foreign_key, and inverse_of, for purposes of instruction; however, it is worth mentioning that they weren’t strictly necessary in this example. If the attribute name (quiz) is the lowercase form of the class name (Quiz) and the column name (quiz_id) is the lowercase form of the class name (Quiz) with an “_id” on the end, then the class_name, foreign_key and inverse_of options do not need to be explicitly declared. Thus, the following belongs_to declaration would be equivalent to the one above:

belongs_to :quiz

Add the following has_many declaration to the Quiz model class, as follows:

has_many(
  :mc_questions, # Quiz attribute containing an array of McQuestion objects
  class_name: 'McQuestion', # datatype of attribute
  foreign_key: 'quiz_id', # name of column containing FK in other table
  inverse_of: :quiz, # attribute on other side of association (parent Quiz object)
  dependent: :destroy # if a quiz is destroyed, also destroy all of its questions
)

Similar to the belongs_to declaration above, the class_name, foreign_key, and inverse_of options are included here for instructional purposes and are not actually required in this case. Thus, the following declaration could suffice:

has_many :mc_questions, dependent: :destroy

Confirm that the newly added code is working as expected by running the model tests; however, be aware that adding the model association has left our test fixtures invalid. For example, try running these commands:

rails db:migrate:reset RAILS_ENV=test
rails test

The test checking that the fixtures are valid should fail with the following error message:

Failure:
McQuestionTest#test_fixtures_are_valid [/Users/sdf/workspace/quiz-me/test/models/mc_question_test.rb:29]:
["Quiz must exist"]

If we see any errors other than this one, then we have accidentally introduced bugs that need to be fixed.

The association we created requires that each McQuestion object must belong to a Quiz object, and it creates an implicit validation that the quiz attribute must be present. This explains why we get a failure message like the following, stating “Quiz must exist”. In the subsequent steps, we will correct this failure by updating the fixtures so that all association links are present.

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3. Updating the McQuestion Model Test Fixtures to Use the Association

Now that the one-to-many association has been set up, we will use it to create some linked Quiz and McQuestion objects in the app’s test fixtures.

Note that the Quiz fixtures in test/fixtures/quizzes.yml do not need to be updated, because it is the McQuestion model class that has the foreign-key attribute.

In test/fixtures/mc_questions.yml, add a quiz attribute to each McQuestion fixture that points to the Quiz fixture we just created in the previous step, like this:

one:
    question: By default, every Rails model is a subclass of which superclass?
    answer: ApplicationRecord
    distractor_1: Object
    distractor_2: ActiveModel
    quiz: one

two:
    question: The command rails db:migrate updates the schema.rb file.
    answer: true
    distractor_1: false
    distractor_2: # blank loads as nil
    quiz: one

In the above code, quiz: one refers to the Quiz fixture in test/fixtures/quizzes.yml named one.

Run the following command to verify that the tests now pass:

rails test

We should see 0 failures and 0 errors. If we do see failures or errors, then there is a bug in the code we added that needs fixing.

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4. Updating the McQuestion Seed Data to Use the Association

As a final task, we will seed the database with example data using our newly created model class association, as per the steps below. In a later demo, we will add pages to view and manipulate this seed data in the app.

Add a three more McQuestion objects, like this:

q4 = McQuestion.create!(
    question: 'Which hash is primarily used for one way message passing from the controller to the view?',
    answer: 'flash',
    distractor_1: 'session',
    distractor_2: 'params'
)

q5 = McQuestion.create!(
    question: 'In which folder are image assets for the QuizMe app stored?',
    answer: 'quiz-me/app/assets/images',
    distractor_1: 'quiz-me',
    distractor_2: 'quiz-me/images'
)

q6 = McQuestion.create!(
    question: 'Which standard RESTful controller action is used to remove records?',
    answer: 'destroy',
    distractor_1: 'delete',
    distractor_2: 'remove'
)

Create association links between the Quiz and McQuestion objects by setting a quiz option in each call to McQuestion.create!, like this:

q1 = McQuestion.create!(
    quiz: quiz1,
    question: 'What does the M in MVC stand for?',
    answer: 'Model',
    distractor_1: 'Media',
    distractor_2: 'Mode'
)

q2 = McQuestion.create!(
    quiz: quiz1,
    question: 'What does the V in MVC stand for?',
    answer: 'View',
    distractor_1: 'Verify',
    distractor_2: 'Validate'
)

q3 = McQuestion.create!(
    quiz: quiz1,
    question: 'What does the C in MVC stand for?',
    answer: 'Controller',
    distractor_1: 'Create',
    distractor_2: 'Code'
)

q4 = McQuestion.create!(
    quiz: quiz2,
    question: 'Which hash is primarily used for one way message passing from the controller to the view?',
    answer: 'flash',
    distractor_1: 'session',
    distractor_2: 'params'
)

q5 = McQuestion.create!(
    quiz: quiz2,
    question: 'In which folder are image assets for the QuizMe app stored?',
    answer: 'quiz-me/app/assets/images',
    distractor_1: 'quiz-me',
    distractor_2: 'quiz-me/images'
)

q6 = McQuestion.create!(
    quiz: quiz2,
    question: 'Which standard RESTful controller action is used to remove records?',
    answer: 'destroy',
    distractor_1: 'delete',
    distractor_2: 'remove'
)

Reset and seed the database using the following command:

rails db:migrate:reset db:seed

To confirm that the data was seeded correctly, use pgAdmin to inspect the database.

Our model now includes association links between quizzes and the multiple-choice questions that belong to them. In the next demo, we will add features to our pages and forms for managing these association links.

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