On Aug 16, 2014, at 6:03 AM, Ronald Fischer wrote:
> (Rails 4, Ruby 2)
>
> I have two functions which query the database. Both yield a set of model
> objects. These two sets should be catenated and presented to the user
> using the will_paginate Gem. I think I understand now how to do it, but
> a few issues with this are still puzzling me.
>
> Here are the query functions:
>
> def the_other_dicts_of_same_user(d)
> User.find_by_id(d.user_id).dicts.where("id != #{d.id}")
> end
>
> def public_dicts_of_other_users(d)
> Dict.where("user_id != #{d.user_id} and world_readable")
> end
>
> Both yield a series of Dict objects. The result of the first query is of
> type Dict::ActiveRecord_AssociationRelation and the result of the second
> query is of type Dict::ActiveRecord_Relation
>
> I guess I get different data types here, because I start with User in
> the first case and with Dict in the second case.
>
> As a side note: I *could* have written the first query alternatively as
>
> Dict.where("user_id = #{d.user_id} and id != #{d.user_id}")
>
> Would this be better?
>
> Anyway, I catenate the results,
>
> @dicts=the_other_dicts_of_same_user(d)+public_dicts_of_other_users(d)
>
> which yields again a Dict::ActiveRecord_Relation . Now I apply
> pagination:
>
> @dicts=@dicts.paginate(....)
>
> This yields the error message, that paginate is not defined for objects
> of type Array.
>
> I was able to get around this error, by doing a
>
> require 'will_paginate/array'
>
> as a first line in my controller, but I wonder: Why do I get this error?
> @dicts is NOT an array (I logged @dicts.class.to_s to be sure of this),
> and I would have expected to work it in a ActiveRecord_Relation.
>
> Could someone please kindly explain this observed behaviour to me?
I suspect that it is because a Relation is not an Array, it's more of a scope.
It can be iterated over, because it extends Enumerable, but that does not make
it an array. But adding the two together must (I am guessing here) cause them
to both be evaluated as arrays before the addition can succeed. When you do
to_a on a Relation (either kind) it loses its magic lazy-loading behavior, and
the elements are found and placed into the array. The whole point of a Relation
is that the found records inside it are not loaded into memory until they are
actually asked for more formally. This makes it much faster (and allows for
further chaining) in a scope.
Walter
>
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