Add column to find

Rails
1

I have a find with a has_many relationship using :include for the
relationship. My question is how do i add an additional column to the
query
and keep all the associations in tact? I need to add a distance column
in
the output for some geocoding but can not figure out how to do this
without
being forced to find_by_sql.

Any ideas would be greatly appreciated.

Thanks
Adam

2

I’ll assume that the “column” in question isn’t in any table but being
computed on the fly. If so, simply modify the ‘:select =>’ parameter to
add your additional column. I.e.

Model.find(:all, :select => ‘*, insert_ugly_trig_formula_here AS
distance’, :conditions => etc. etc.)

Notice the ‘*,’ so that you still get the other columns you (and
ActiveRecord) normally expect. Once the query returns you’ll have an
@object.distance method you can call that gives you the computed
distance for that record.

3

Thanks Jon.

Does this work as well with eager loading ? Thats why I included that
i
was using :include as well in my find. Just want to make sure that is
not
going to throw this in a hissy, because i have been trying :select with
eager loading and not having any luck.

Thanks
Adam

4

Actually, no. Eager loading with :include => overwrites anything you put
in the :select => parameter, so you can’t do both. Sorry, I didn’t pick
up on the importance of the eager loading in your original question.

5

Thats what I thought. Any suggestions on a workaround? The only other
thing I was thinking was somehow using a postgresql function or view to
make
an additional column available at the postgres level.

Thanks again for the help.
Adam

6

The one time I’ve tried to do this, I didn’t actually need to eager
load, I just needed to include other tables in the SELECT function and
the WHERE conditions, so I used :joins => instead of :include => and
that worked well for me. That may not work for you though. Your other
options are, A) not eager load, or B) compute ‘distance’ for each item
later on in a separate method.

7

Yes, :select and :joins play very nice together.

8

Jon,

So does that mean i can use :select with :joins , but not :include ?

The function idea is a good recommendation, but I unfortunately need to
sort the sql query by distance .

Thanks
Adam