has_one relationship in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the has_one relationship represent in Rails?

It means one model owns or is linked to exactly one instance of another model. For example, a User has one Profile.

beginner

How do you declare a has_one association in a Rails model?

Inside the model class, use has_one :associated_model. For example, has_one :profile inside User model.

intermediate

What is the difference between has_one and belongs_to in Rails?

has_one is used in the model that owns the other, while belongs_to is used in the model that holds the foreign key pointing back. They work together to link two models.

intermediate

Where is the foreign key stored in a has_one relationship?

The foreign key is stored in the table of the model that belongs_to the other. For example, if User has_one Profile, the profiles table has a user_id column.

intermediate

How does Rails handle dependent objects in a has_one association?

You can add dependent: :destroy to automatically delete the associated object when the owner is deleted, keeping data clean.

In a Rails has_one association, which model contains the foreign key?

A Neither model has a foreign key

B The model with <code>has_one</code>

C Both models have the foreign key

D The model with <code>belongs_to</code>

The foreign key is stored in the model that uses belongs_to, pointing back to the owner.

Which method declaration correctly sets up a has_one association in Rails?

A has_one :profile

B belongs_to :profile

C has_many :profile

D has_one_of :profile

Use has_one :profile to declare a one-to-one association where the model owns one profile.

What happens if you add dependent: :destroy to a has_one association?

A Nothing happens automatically

B The associated object is deleted when the owner is deleted

C The owner is deleted when the associated object is deleted

D Both objects are deleted simultaneously

Adding dependent: :destroy deletes the associated object automatically when the owner is removed.

If a User model has has_one :profile, how do you access the profile of a user instance?

A user.profile

B user.profiles

C profile.user

D profile.users

You call user.profile to get the single profile linked to that user.

Which of these best describes the has_one relationship?

A Models are unrelated

B One model owns many instances of another model

C One model owns exactly one instance of another model

D Many models belong to one model

has_one means one model owns exactly one instance of another model.

Explain how the has_one relationship works in Rails and where the foreign key is stored.

Think about which model points to the other with an ID.

Describe how to set up a has_one association with dependent destruction in Rails.

Consider what happens when you delete the main object.