Specify fields you want on FactoryBot - ruby-on-rails

Question

I have an object that takes the information from a rest API. Let's say I call Posts from an API and I can specify which fields I want to extract. Just an example of how it looks:
Post.find!(id: 1, fields: [:id, :title, :description])
This does an API call and it returns me a Post object with these specified fields.
For testing, I am stubbing this API call with Factory Bot and returns directly a Post object with all possible fields it can query.
This approach is not the best since the tests are always returning all fields and code itself maybe I just need a couple of fields not all of them
So I am trying to achieve something like (in FactoryBot):
build(:post, fields: [:id,:title]) and set up a Post object just with id and title.
If I do build(:post, fields: [:title, :created_at]) and set up a Post object just with title and created_at. And so on...
Did some research and trying some ideas, but failed in all of them about how to build this flow.
Any idea about how to achieve this behavior?
EDIT
Traits seems a nice alternative, but I must be as consistent as the API call, specifying these fields. So traits are not working for me...

Answer 1

Let's assume that this is your factory for the Post:
FactoryBot.define do
factory :post do
sequence(:title) { |n| "Post no. #{n}" }
description 'Post description'
created_at { DateTime.now }
end
end
When you call build(:post) it will create an object with title, created_at and description set.
But if you will remove those fields from your factory (or move them under trait):
FactoryBot.define do
factory :post do
trait :all_fields do
sequence(:title) { |n| "Post no. #{n}" }
description 'Post description'
created_at { DateTime.now }
end
end
end
Then:
calling build(:post, title: 'Post title', description: 'Post description') will create a post object where only title and description are set (created_at will be nil)
calling build(:post, :all_fields) will create a post object will all fields set.
Edit
I think I understand the problem better now. Let's assume that this is you factory:
FactoryBot.define do
factory :post do
sequence(:title) { |n| "Post no. #{n}" }
created_at { DateTime.now }
description 'Post description'
end
end
Change it to:
FactoryBot.define do
factory :post do
transient do
fields []
end
sequence(:title) { |n| "Post no. #{n}" }
created_at { DateTime.now }
description 'Post description'
after :build do |post, evaluator|
unless evaluator.fields.empty? do
(post.attributes.keys - evaluator.fields).each do |attribute_to_remove|
post.send("#{attribute_to_remove}=", nil)
end
end
end
end
end
Then, you can call it like this:
build(:post) creates post with all fields
build(:post, fields: ['description', 'title']) creates a post where everything except description and title is nil.
This solution should work as expected but it might slow down your tests (and I think it does not look nice :) )

Answer 2

FactoryBot lets you override the factory by passing a hash of attributes - so why not just set the attributes to nil:
build(:post, {
title: nil,
description: nil
})

Answer 3

Best practice in FactoryBot is to have your standard factory produce objects that contain only the required fields for a model. (Note that FactoryBot will generate an id for you automatically.)
Assume you have a model called Post that requires only a title, but has optional fields description and date. Your factory would look like this:
FactoryBot.define do
factory :post do
title 'Post Title'
end
end
Now when you build a Post, it will look like this:
>> FactoryBot.build_stubbed(:post)
#> <Post id: 1001, title: "Post Title", description: nil, date: nil>
You can add the optional fields on a case-by-case basis:
>> FactoryBot.build_stubbed(:post, description: 'This is a test post.')
#> <Post id: 1001, title: "Post Title", description: 'This is a test post.', date: nil>
Or you can add traits within the factory:
FactoryBot.define do
factory :post do
title 'Post Title'
trait :with_description_and_date do
description 'This is a test post.'
date Date.today
end
end
end
>> FactoryBot.build_stubbed(:post, :with_description_and_date)
#> <Post id: 1001, title: "Post Title", description: 'This is a test post.', date: Thu, 12 Apr 2018>