Limit on eager loaded association?

Rails
1

If I use :include to eager load a has_many association, can I limit the
number returned on that has_many association. I know I can limit the
amount of records returned, but how do I limit how many are returned on
its association?

2

Hi,

If I use :include to eager load a has_many association, can I limit the
number returned on that has_many association. I know I can limit the
amount of records returned, but how do I limit how many are returned on
its association?
If there is a way of limiting the includes explicitely when using the
finder, I’m not aware of it.But you can use :limit as one of the options
when you are defining the has_many association itself at the model, so
you could so something like:

has_many :detail_items, :limit=>10

There are also a number of other options you can use to further
selecting which of the detail_items will be retrieved for this
association, like :order or :select.

Regards,

javier ramirez

3

ActiveRecord can’t currently do that, but it’d be a nice feature given
how difficult it is to customize the eager loading queries.

However, you might be able emulate the functionality by constructing
your own query to fetch the ids of the association that match. Then
you could put them in a big array and pass them into the conditions on
the final query.

There would be a lot of issues, and it might ultimately be slower than
the n+1 queries. There’s no direct way to limit the secondary results
although you might be able to cook something up with subselects or
other advanced db features.

On May 15, 4:03 pm, Aryk G. [email protected]

4

I was thinking about that Javier, but I think it would ignore the :limit
when it gets eager loaded.

Have you heard of it working?

5

Aryk G. wrote:

I was thinking about that Javier, but I think it would ignore the :limit
when it gets eager loaded.

to be honest i assumed it would work when eager loading but didn’t try
before. I’m successfully using it for a model in which i’m not using
eager-loading (just for a combo with ajax, so no need for the details
beforehand) and I thought it would be just fine when including. I just
tested that on the console and you are right, it will totally ignore the
limit option in that case.

I understand the sql for limiting on the association would get a bit
tricky, but I was hoping AR would apply the limit when mapping the
results. Unfortunately it’s not.

regards,

javier