How to retrieve values from a column called "object_id" in ActiveRecord?

Question

I need to access a legacy relational database via ActiveRecord and that database uses a column named "object_id" as a primary key in a table. Most things work but when I try specify a custom SQL query for an association (see below) ActiveRecord respectively the Ruby interpreter always retrieves the object_id of the Ruby base "Object" instead of the column value from the database.

I defined the following SQL query in a has_many declaration:

:finder_sql => "SELECT * FROM t_object WHERE object_id IN (SELECT end_object_id FROM t_object, t_connector WHERE t_object.object_id = #{object_id} AND start_object_id = #{object_id})

I already tried to solve it by working with an alias:

alias_attribute :my_oid, :object_id

and using "my_oid" in the SQL query. But I got the following error message:

NameError: undefined local variable or method `my_oid' for EaTObject(Table doesn't exist):Class

I also defined a method which returns the value of read_attribute("object_id") and put the method name into the SQL query instead of "object_id" but when the class was loaded I got the error message that that method name is not a column name in the database table.

Is there any solution to access a column called "object_id"?

Answer 1

You can use this method:

read_attribute(:object_id)

The link in documentation :

http://api.rubyonrails.org/classes/ActiveRecord/AttributeMethods/Read.html#method-i-read_attribute

So if you want access to your object_id column whan you call oid you need delete your alias and define the oid method

def oid
  read_attribute(:object_id)
end

Answer 2

You may use t_object.object_id in your SQL to specify object_id field, or you may alias your t_object table, e.g.

:finder_sql => "SELECT * FROM t_object AS t WHERE t.object_id IN..."

Answer 3

Another option is self[:object_id], but if you are getting "Table doesn't exist", then maybe there is something else wrong?

Edit: Try this:

:finder_sql => 'SELECT * FROM t_object WHERE object_id IN (SELECT end_object_id FROM t_object, t_connector WHERE t_object.object_id = #{id} AND start_object_id = #{id})'

NB! single quotes.