How can I permanently ignore a database column in my ActiveRecord::Base class?

Question

I have a legacy database which I\'m trying to model using Rails. One of the tables has a column named attributes, which is a name reserved by Rails I think.

Answer 1

  class << self
     def instance_method_already_implemented?(method_name)
      return true if method_name =~ /^attributes/
      super
    end
  end

This patch is fine and was working for me mostly, but when you check something like Album.column_names and Album.columns.collect(&:name) you would still get all columns. Also this would fail.

 a = Album.last
 a = Album.new(a.attributes) 

To have rails fully ignore a column you could do this.

 class Album < ActiveRecord::Base
    set_table_name "album"
    ## --------------------- Ignore columns patch ------
    @@ignore_column_pattern = /^column_one|^column_two/

    class << self
      alias :all_columns :columns
      def columns 
        @columns_filt ||= all_columns.reject { |col| col.name =~ @@ignore_column_pattern } 
      end 
    end

    alias :all_attribute_names :attribute_names
    def attribute_names
      @attr_names_filt ||= all_attribute_names.reject { |att| att =~ @@ignore_column_pattern }
    end
    ## ------------------- / Ignore columns patch ------
    belongs_to :artist
    has_many :tracks, :through => :album_tracks
  end

Also I used an array to store the columns I didn't want but you could still use the regex way too! Now the only way that you can find out about the column is using connection ie.

Album.connection.columns("albums").collect(&:name)

Answer 2

(This is an old question but still figures in Google so I'll add a belated answer)

I've found the best way to restrict data loaded by an ActiveRecord model is to create a database view that only contains the columns you wish to load. You can point your ActiveRecord model at the constrained view using ActiveRecord's table_name method. Native SQL tools can still manipulate the underlying table, but ActiveRecord will only see (and consequently load) the columns explicitly included in the view.

Answer 3

Solved this using a combination of stuff from Robin's link and some other SO answers

class Album < ActiveRecord::Base
  set_table_name "album"

  class << self
    def instance_method_already_implemented?(method_name)
      return true if method_name =~ /^attributes/
      super
    end
  end

  belongs_to :artist
  has_many :tracks, :through => :album_tracks
end

Did the trick. I used a big sweeping change to return true without throwing an error for all methods starting with attributes, and I don't think its caused any problems elsewhere.