Validate that an object has one or more associated objects

Question

I need to ensure that when a product is created it has atleast one category. I could do this with a custom validation class, but I was hoping there was a more standard way of doing it.

class Product < ActiveRecord::Base
  has_many :product_categories
  has_many :categories, :through => :product_categories #must have at least 1
end

class Category < ActiveRecord::Base
  has_many :product_categories
  has_many :products, :through => :product_categories
end

class ProductCategory < ActiveRecord::Base
  belongs_to :product
  belongs_to :category
end

1. products + categories is a great opportunity to meet `has_and_belongs_to_many` http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many. You don't need join model unless you don't want to store additional attributes alongside association. 2. You can use top answer from this question http://stackoverflow.com/questions/6429389/how-can-i-make-sure-my-has-many-will-have-a-size-of-at-least-2 guess what you have to change :) — jibiel, Mar 02 '12 at 15:36

score 73 · Accepted Answer · edited May 31 '16 at 15:06

73

There is a validation that will check the length of your association. Try this:

class Product < ActiveRecord::Base
  has_many :product_categories
  has_many :categories, :through => :product_categories

  validates :categories, :length => { :minimum => 1 }
end

edited May 31 '16 at 15:06

davegson

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answered Mar 02 '12 at 15:35

wpgreenway

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How do I write a spec to test this ? – abhishek77in Mar 11 '15 at 11:02
@abhishek77in I found something that said to use `it {should validate_length_of(:categories).is_at_least(1)}` but I get an error saying undefined method each for string. I was thikning that testing for presence might do the trick as it would require at least one record in there. – Int'l Man Of Coding Mystery May 26 '18 at 13:32

score 60 · Answer 2 · edited May 31 '16 at 15:07

60

Ensures it has at least one category:

class Product < ActiveRecord::Base
  has_many :product_categories
  has_many :categories, :through => :product_categories

  validates :categories, :presence => true
end

I find the error message using :presence is clearer than using length minimum 1 validation

edited May 31 '16 at 15:07

davegson

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answered Jul 14 '14 at 19:28

Adrian

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"can't be blank" definitely makes more sense than "is too short (minimum is 1 character)" – mzrnsh Jul 09 '22 at 10:46

score 4 · Answer 3 · edited May 31 '16 at 15:08

Instead of wpgreenway's solution, I would suggest to use a hook method as before_save and use a has_and_belongs_to_many association.

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
  before_save :ensure_that_a_product_belongs_to_one_category

  def ensure_that_a_product_belongs_to_one_category
    if self.category_ids < 1 
      errors.add(:base, "A product must belongs to one category at least")
      return false
    else
      return true
    end
  end   

class ProductsController < ApplicationController
  def create
    params[:category] ||= []
    @product.category_ids = params[:category]
    .....
  end
end

And in your view, use can use for example options_from_collection_for_select

Validate that an object has one or more associated objects

3 Answers3

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