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POCorrect way to handle multiparameter attributes corresponding to virtual attributes
 

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    <p>I have a Rails app with a model containing a <code>birthdate</code> attribute. This corresponds to a column in my database defined using the ActiveRecord <code>date</code> type. With this I am able to use the <code>date_select</code> form helper method to render this as a three-select input in my view. The form parameters corresponding to this field are then serialized back to the controller as <code>birthdate(1i)</code>, <code>birthdate(2i)</code> and <code>birthdate(3i)</code>. Consequently, I can use the standard <code>update_attributes</code> method within my controller on my model to update all fields on my model.</p> <p>I'm now experimenting with encrypting this field using the <a href="https://github.com/shuber/attr_encrypted" rel="nofollow"><code>attr_encrypted</code></a> gem. While the gem supports marshalling (this is nice), there is no longer a real column of name <code>birthdate</code> of type <code>date</code> - instead, <code>attr_encrypted</code> exposes the value as a <em>virtual</em> attribute <code>birthdate</code> backed by a real <code>encrypted_birthdate</code> column. This means that <code>update_attributes</code> is unable to perform the previous multiparameter attribute assignment to populate and save this column. Instead, I get a <code>MultiparameterAssignmentErrors</code> error resulting from the call to the internal <code>column_for_attribute</code> method returning <code>nil</code> for this column (from somewhere within <code>execute_callstack_for_multiparameter_attributes</code>).</p> <p>I'm currently working around this as follows:</p> <p>My model in <code>app/models/person.rb</code>:</p> <pre><code>class Person &lt; ActiveRecord::Base attr_encrypted :birthdate end </code></pre> <p>My controller in <code>app/controllers/people_controller.rb</code>:</p> <pre><code>class PeopleController &lt; ApplicationController def update # This is the bit I would like to avoid having to do. params[:person] = munge_params(params[:person]) respond_to do |format| if @person.update_attributes(params[:person]) format.html { redirect_to @person, notice: 'Person was successfully updated.' } format.json { head :no_content } else format.html { render action: "edit" } format.json { render json: @person.errors, status: :unprocessable_entity } end end end private def munge_params(params) # This separates the "birthdate" parameters from the other parameters in the request. birthdate_params, munged_params = extract_multiparameter_attribute(params, :birthdate) # Now we place a scalar "birthdate" where the multiparameter attribute used to be. munged_params['birthdate'] = Date.new( birthdate_params[1], birthdate_params[2], birthdate_params[3] ) munged_params end def extract_multiparameter_attribute(params, name) # This is sample code for demonstration purposes only and currently # only handles the integer "i" type. regex = /^#{Regexp.quote(name.to_s)}\((\d+)i)\)$/ attribute_params, other_params = params.segment { |k, v| k =~ regex } attribute_params2 = Hash[attribute_params.collect do |key, value| key =~ regex or raise RuntimeError.new("Invalid key \"#{key}\"") index = Integer($1) [index, Integer(value)] end] [attribute_params2, other_params] end def segment(hash, &amp;discriminator) hash.to_a.partition(&amp;discriminator).map do |a| a.each_with_object(Hash.new) { |e, h| h[e.first] = e.last } end end end </code></pre> <p>And my view <code>app/views/people/_form.html.erb</code>:</p> <pre><code>&lt;%= form_for @person do |f| %&gt; &lt;%= f.label :birthdate %&gt; &lt;%= f.date_select :birthdate %&gt; &lt;% f.submit %&gt; &lt;% end %&gt; </code></pre> <p>What's the proper way to handle this type of attribute without having to introduce ad hoc munging of the <code>params</code> array like this?</p> <p>Update: Looks like <a href="https://rails.lighthouseapp.com/projects/8994/tickets/2675-support-for-multiparameter-attribute-assignment-on-virtual-attribute-writers" rel="nofollow">this</a> might refer to a related problem. And <a href="https://rails.lighthouseapp.com/projects/8994/tickets/2675-support-for-multiparameter-attribute-assignment-on-virtual-attribute-writers" rel="nofollow">this</a> too.</p> <p>Another update:</p> <p>Here is my current solution, based on Chris Heald's answer. This code should be added to the <code>Person</code> model class:</p> <pre><code>class EncryptedAttributeClassWrapper attr_reader :klass def initialize(klass); @klass = klass; end end # TODO: Modify attr_encrypted to take a :class option in order # to populate this hash. ENCRYPTED_ATTRIBUTE_CLASS_WRAPPERS = { :birthdate =&gt; EncryptedAttributeClassWrapper.new(Date) } def column_for_attribute(attribute) attribute_sym = attribute.to_sym if encrypted = self.class.encrypted_attributes[attribute_sym] column_info = ENCRYPTED_ATTRIBUTE_CLASS_WRAPPERS[attribute_sym] column_info ||= super encrypted[:attribute] column_info else super end end </code></pre> <p>This solution works as is, but would be even better if <code>attr_encrypted</code> were to take a <code>:class</code> option that would construct the <code>ENCRYPTED_ATTRIBUTE_CLASS_WRAPPERS</code> hash dynamically. I'm going to look at ways I can extend/monkeypatch <code>attr_encrypted</code> to do this. Gist available here: <a href="https://gist.github.com/rcook/5992293" rel="nofollow">https://gist.github.com/rcook/5992293</a>.</p>
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