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Wednesday, January 18, 2012

Getting the list of aliased key/attribute names from a Mongoid model

At some point today when I was writing some model specs for one of my Mongoid models, I required the list of all of the attribute/key names. Mongoid provides a handy "fields" method for this, which returns an hash of key names and Mongoid::Fields::Serializable object pairs. Getting the list of names from that was easy : Model.fields.keys.

This gives the list of the actual key names. The actual key names, in my case, are very short strings (1 to 3 characters) and I have long aliases for those in my models. What I eventually realized was that I wanted the list of  the longer aliased names. Looking around the mongoid code did not give me any direct method. Turns out that the aliased names result in nothing more than a few additional 'wrapper' methods (like the accessors, dirty methods, etc) and there is no table/hash kind of thing maintained anywhere to give the mapping between the actual key name and the aliased ones. So my current guess is that the list of these aliased names is not available directly anywhere.

So I came up with this hackish way of getting that list of aliased names.

p = Post.new
actual_field_names = p.fields.keys
all_field_names = p.methods.collect { |m| m.to_s.match(/_changed\?$/).try(:pre_match) }
                    .select { |e| e }
aliased_field_names = all_field_names - actual_field_names

As mentioned earlier, this is pretty hackish. If you know of a straight forward way, do let me know.

Note : I eventually found out that I did not actually need this list of aliased names. I did not use this in my project. Nevertheless it works just fine.

1 comment:

  1. Thanks. I used you method and it works well, but...

    I also looked at the collected list of methods. In there, I found the method


    It returns a hash with the aliased name as key and the field name as a value.

    If you want to create multiple aliases you can do this (in a callback):

    more = {
    "new_alias_key" => "field_name"
    self.aliased_fields = aliased_fields.merge(more)