On Nov 22, 2014, at 5:03 PM, Sunny Juneja <jr.su...@gmail.com> wrote:
> Hey everyone,
>
> I've obviously done a bad job of explaining this :). Let me try to elaborate.
>
> Suppose I've ran xyz = Model.where(attribute: true)
> A SQL query is created and executed. The results are saved to xyz as an
> ActiveRecord_Relation. The query looks something like this:
> SELECT "models".* FROM "models" WHERE "models"."attribute" = ’t'
This is a place that people frequently get confused - doing this only runs the
query in the REPL, where the result has `inspect` called on it for printing.
Try this in the REPL to see the difference:
xyz = Model.where(attribute: true); nil
This will set `xyz` but not trigger the query. Any subsequent code that
attempts to access elements of `xyz` will trigger the query - so `to_a`, or
`each`, etc.
Note that some methods will trigger a different query - for instance, calling
`xyz.first` will perform a query with `LIMIT 1` if the records haven’t been
loaded yet.
> If I run xyz.find_by(attribute2: false), a nw SQL query is created and
> executed and it will hit the database again. The query looks something like
> this:
> SELECT "models".* FROM "models" WHERE "models"."attribute" = 't' AND
> "models"."attribute2" = 'f' LIMIT 1
> On the other hand, the proposed find_in_relation is a method on an
> ActiveRecord_Relation that doesn't hit the database and searches the relation
> in memory.
>
> If I type xyz.find_in_relation(attribute2: false), no new sql query is
> created and it will not touch the database. The code would be equivalent to
> running:
> xyz.select { |x| x.attribute2 == false }
>
> Ideally, find_in_relation would return another ActiveRecord_Relation.
>
`Relation` is intended to build queries. The results of in-memory filtering are
not going to make that possible, in general, short of flattening everything to
a giant `id IN (?)` clause.
The only use case I can see for something like this is when some of the
attributes you’re filtering by aren’t DB columns but instead are model methods.
For that case, `select` seems
to make the in-memory filtering aspect clearer.
—Matt Jones
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