We moved more and more away from passing options to finder / calculation
methods. The `:distinct` option in `#count` was one of the remaining places.
Since we can now combine `Relation#distinct` with `Relation#count` the option
is no longer necessary and can be deprecated.
The similarity of `Relation#uniq` to `Array#uniq` is confusing. Since our
Relation API is close to SQL terms I renamed `#uniq` to `#distinct`.
There is no deprecation. `#uniq` and `#uniq!` are aliases and will continue
to work. I also updated the documentation to promote the use of `#distinct`.
This caused a bug with the new associations implementation, because now
association conditions are represented as Arel nodes internally right up
to when the whole thing gets turned to SQL.
In Rails 3.2, association conditions get turned to raw SQL early on,
which prevents Relation#merge from interfering.
The current implementation was buggy when a default_scope existed on the
target model, since we would basically end up doing:
default_scope.merge(association_scope)
If default_scope contained a where(foo: 'a') and association_scope
contained a where(foo: 'b').where(foo: 'c') then the merger would see
that the same column is representated on both sides of the merge and
collapse the wheres to all but the last: where(foo: 'c')
Now, the RHS of the merge is left alone.
Fixes#8990
At present, ActiveRecord::Delegation compiles delegation methods on a
global basis. The compiled methods apply to all subsequent Relation
instances. This creates several problems:
1) After Post.all.recent has been called, User.all.respond_to?(:recent)
will be true, even if User.all.recent will actually raise an error due
to no User.recent method existing. (See #8080.)
2) Depending on the AR class, the delegation should do different things.
For example, if a Post.zip method exists, then Post.all.zip should call
it. But this will then result in User.zip being called by a subsequent
User.all.zip, even if User.zip does not exist, when in fact
User.all.zip should call User.all.to_a.zip. (There are various
variants of this problem.)
We are creating these compiled delegations in order to avoid method
missing and to avoid repeating logic on each invocation.
One way of handling these issues is to add additional checks in various
places to ensure we're doing the "right thing". However, this makes the
compiled methods signficantly slower. In which case, there's almost no
point in avoiding method_missing at all. (See #8127 for a proposed
solution which takes this approach.)
This is an alternative approach which involves creating a subclass of
ActiveRecord::Relation for each AR class represented. So, with this
patch, Post.all.class != User.all.class. This means that the delegations
are compiled for and only apply to a single AR class. A compiled method
for Post.all will not be invoked from User.all.
This solves the above issues without incurring significant performance
penalties. It's designed to be relatively seamless, however the downside
is a bit of complexity and potentially confusion for a user who thinks
that Post.all and User.all should be instances of the same class.
Benchmark
---------
require 'active_record'
require 'benchmark/ips'
class Post < ActiveRecord::Base
establish_connection adapter: 'sqlite3', database: ':memory:'
connection.create_table :posts
def self.omg
:omg
end
end
relation = Post.all
Benchmark.ips do |r|
r.report('delegation') { relation.omg }
r.report('constructing') { Post.all }
end
Before
------
Calculating -------------------------------------
delegation 4392 i/100ms
constructing 4780 i/100ms
-------------------------------------------------
delegation 144235.9 (±27.7%) i/s - 663192 in 5.038075s
constructing 182015.5 (±21.2%) i/s - 850840 in 5.005364s
After
-----
Calculating -------------------------------------
delegation 6677 i/100ms
constructing 6260 i/100ms
-------------------------------------------------
delegation 166828.2 (±34.2%) i/s - 754501 in 5.001430s
constructing 116575.5 (±18.6%) i/s - 563400 in 5.036690s
Comments
--------
Bear in mind that the standard deviations in the above are huge, so we
can't compare the numbers too directly. However, we can conclude that
Relation construction has become a little slower (as we'd expect), but
not by a huge huge amount, and we can still construct a large number of
Relations quite quickly.
This is a real fix (as compared to the band-aid in b127d86c), which uses
the recently-added equality methods for ARel nodes. It has the side
benefit of simplifying the merge code a bit.
This is at best a band-aid for a more proper fix, since it won't truly
handle the removal of the previous equality condition of these other
nodes. I'm planning to put in some work on ARel toward supporting that
goal.
Related: rails/arel#130, ernie/squeel#153, ernie/squeel#156
This method explicitly loads the records and then returns `self`.
Rather than deciding between "do I want an array or a relation?",
most people are actually asking themselves "do I want to eager load
or lazy load?" Therefore, this method provides a way to explicitly
eager-load without having to switch from a `Relation` to an array.
Example:
@posts = Post.where(published: true).load
User.order("name asc").order("created_at desc")
# SELECT * FROM users ORDER BY created_at desc, name asc
This also affects order defined in `default_scope` or any kind of associations.
Previously it returned an Array.
If you want an array, call e.g. `Post.to_a` rather than `Post.all`. This
is more explicit.
In most cases this should not break existing code, since
Relations use method_missing to delegate unknown methods to #to_a
anyway.
While it's interesting to have the results array, it can make a console or a webpage freeze if there are a lot of them.
So this limits the number of records displayed in #inspect to 10 and tells how much were effectively found.
The reason for removing the previous implementation of `#inspect` was
that it hid from you that you were dealing with a `Relation` rather than
an `Array`.
But it is still useful to be able to see the records, particularly if you're
writing something like the following in tests:
assert_equal [foo], Post.where(:bar)
If the assertion fails, you want to see what records were actually
loaded.
So this implementation makes it clear that you've got a `Relation`, but
also shows your records.
