---
title:
  "Rails 7 allows constructors (build_association and create_association) on
  has_one :through associations"
description:
  "Rails 7 allows constructors (build_association and create_association) on
  has_one:through associations"
canonical_url: "https://www.bigbinary.com/blog/rails-7-allows-build-and-create-association-constructors-on-has-one-through"
markdown_url: "https://www.bigbinary.com/blog/rails-7-allows-build-and-create-association-constructors-on-has-one-through.md"
---

# Rails 7 allows constructors (build_association and create_association) on has_one :through associations

Rails 7 allows constructors (build_association and create_association) on
has_one:through associations

- Author: Akanksha Jain
- Published: April 6, 2021
- Categories: Rails, Rails 7

<br />

### What are `build_association` and `create_association` constructors?

When we declare either a `belongs_to` or `has_one` association, the declaring
class automatically gains the following methods related to the association:

1. build_association(attributes = {})
1. create_association(attributes = {})

In the above methods \_`association` is replaced with the symbol(association
name) passed as the first argument while declaring the associations. For
example:

```ruby
class Book < ApplicationRecord
  belongs_to :author
end

@book.build_author(name: 'John Doe', email: 'john_doe@example.com')
#=> Returns a new Author object, instantiated with the passed attributes
#=> Links through the book's object foreign key
#=> New author object won't be saved in the database

@book.create_author(name: 'John Doe', email: 'john_doe@example.com')
#=> Returns a new Author object, instantiated with the passed attributes
#=> Links through the book's object foreign key
#=> The new author object will be saved in the database after passing all of the validations specified on the Author model
```

### Before Rails 7

The `build_association` and `create_association` constructors were only
supported by `belongs_to` and `has_one` associations.

Consider the example below. We have a model, **Member**, that has a `has_one`
association with the **CurrentMembership** model. It also has a
`has_one :through` association with the **Club** model.

```ruby
class Member < ApplicationRecord
  has_one :current_membership
  has_one :club, through: :current_membership
end

@member.build_club
#=> NoMethodError (undefined method `build_club' for #<Member:0x00007f9ea2ebd3e8>)
#=> Did you mean?  build_current_membership

@member.create_club
#=> NoMethodError (undefined method `create_club' for #<Member:0x00007f9ea2ebd3e8>)
#=> Did you mean?  create_current_membership
```

### After Rails 7

Users are allowed to use constructors (`build_association` and
`create_association`) on `has_one :through` associations along with `belongs_to`
and `has_one` associations.

```ruby
class Member < ApplicationRecord
  has_one :current_membership
  has_one :club, through: :current_membership
end

@member.build_club
#=> #<Club:0x00007f9ea01a8ce0>

@member.create_club
#=> #<Club:0x00007f9ea01a8ce0>
```

Check out this [pull request](https://github.com/rails/rails/pull/40007) for
more details.

## Links

- [Human page](https://www.bigbinary.com/blog/rails-7-allows-build-and-create-association-constructors-on-has-one-through)