How to Validate Associated Model Id

how to validate associated model id?

You should look into creating a custom validation method:

class Student < ActiveRecord::Base
  validates :course_id, presence: true, numericality: { only_integer: true }
  ...
  validate :validate_course_id

private

  def validate_course_id
    errors.add(:course_id, "is invalid") unless Course.exists?(self.course_id)
  end
end

First, your model will make sure that the course_id is a valid integer, and then your custom validation will make sure that the course exists in the database.

Validating associated models in Rails

Try:

validates :measurement, inclusion: { in: ->(record) { record.food.measurements } }

validates is a method defined on a class, and is being evaluated when a class is declared. Normally, when inclusion values are known before program starts (and are static), passing value is sufficient - validator (which is being created when validates is called) simply saves the passed object and uses it when validating.

In your case the inclusion values are unknown at the point when validator is created (and they also depends on the validating object). Hence, you need to pass a lambda, so validator can use it to get inclusion values during a runtime.

Note also, that validator object is attached to a class, not to a particular instance, so lambda needs to have record param.

How do I validate an association value is present in Rails?

Please try using custom validation

class User < ApplicationRecord
  validate :company_aum_id_present, if: :provider?

  private def company_aum_id_present
    self.errors[:aum_id] << 'Assets Under Management is required.' if company && company.aum_id.blank?
  end
end

Also When usin custom validator you dont need to include ActiveModel::Validations as it is already included by ApplicationRecord

Ruby on Rails - Validate attribute with Associated Model attribute

You will need to add custom validation

class ModelA < ActiveRecord::Base
  has_many: :model_b
end

class ModelB < ActiveRecord::Base
  belongs_to : model_a
  validates :is_between_parent_period

  private

  def is_between_parent_period
    unless start_date.between?(self.model_a.start_date, self.model_a.end_date) && end_date.between?(self.model_a.start_date, self.model_a.end_date)
      errors.add(:base, 'Must be between parent start date and end date')
    end
  end
end

Rails 4: Difference between validates presence on id or association

Investigating further, I found that the 'presence' validator resolves to 'add_on_blank':

http://apidock.com/rails/ActiveModel/Errors/add_on_blank

def add_on_blank(attributes, options = {})
  Array(attributes).each do |attribute|
    value = @base.send(:read_attribute_for_validation, attribute)
    add(attribute, :blank, options) if value.blank?
  end
end

This does what it says: adds a validation error if the property in question is blank?

This means it's simply an existence check. So if I validate an id, that id has to exist. That means:

topping.pancake = Pancake.new
topping.valid?

would return false. However:

topping.pancake_id = -12
topping.valid?

would return true. On the other hand, if I validate the object the exact opposite would be true. Unless -12 is a valid index, in which case ActiveRecord would automatically load it from the database on receipt of the 'pancake' message.

Moving on to my issue, further investigation showed that blank? delegates to empty?, and indeed someone had defined empty? on the pancake, returning true if there are no toppings.

Culprit found, and something about Rails learned.

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