title: Required Params Validations in Rails 5 API
body: <p>Thank you for looking onto this. I have a rails 5 API application. I am using ActiveModel Validations to validate params. 
  I need to validate the keys of params. ie. all the keys are mandatory to keep the structure of request unique, but it can be blank(ie. the values)
  I know the </p>
  
  <pre><code>validates :key presence: true
  </code></pre>
  
  <p>validation, but it is checking there is a value for that. As i said, it can have blank values.</p>
  
  <p>I am using params.permit so that additional keys are not allowed</p>
  
  <pre><code>include ActiveModel::Validations
    validates :key1, presence: true
  def initialize
    @key1 = "val"
    @key2 = "val2"
    params.permit(:key1,:key2)
  end
  </code></pre>
  
  <p>I need to compel the user to do requests with all the parameters with blanks allowed</p>
  
  <p>Thanks in advance</p>