So this bug is kinda funky. The code path is basically "if we weren't passed an
instance of the class we compose to, and we have a converter, call that".
Ignoring the hash case for a moment, everything after that was roughly intended
to be the "else" clause, meaning that we are expected to have an instance of
the class we compose to. Really, we should be blowing up in that case, as we
can give a much better error message than what they user will likely get (e.g.
`NameError: No method first for String` or something). Still, Ruby is duck
typed, so if the object you're assigning responds to the same methods as the
type you compose to, knock yourself out.

The hash case was added in 36e9be85 to remove a bunch of special cased code from
multiparameter assignment. I wrongly assumed that the only time we'd get a hash
there is in that case. Multiparameter assignment will construct a very specific
hash though, where the keys are integers, and we will have a set of keys
covering `1..part.size` exactly. I'm pretty sure this could actually be passed
around as an array, but that's a different story. Really I should convert this
to something like `class MultiParameterAssignment < Hash; end`, which I might
do soon. However for a change that I'm willing to backport to 4-2-stable, this
is what I want to go with for the time being.

Fixes #25978

This behavior was broken by 36e9be85. When the value is assigned
directly, either through mass assignment or directly assigning a hash,
the hash gets passed through to this writer method directly. While this
is intended to handle certain cases, when an explicit converter has been
provided, we should continue to use that instead. The positioning of the
added guard caused the new behavior to override that case.

Fixes #25210

The focus of this change is to make the API more accessible.
References to method and classes should be linked to make it easy to
navigate around.

This patch makes exzessiv use of `rdoc-ref:` to provide more readable
docs. This makes it possible to document `ActiveRecord::Base#save` even
though the method is within a separate module
`ActiveRecord::Persistence`. The goal here is to bring the API closer to
the actual code that you would write.

This commit only deals with Active Record. The other gems will be
updated accordingly but in different commits. The pass through Active
Record is not completely finished yet. A follow up commit will change
the spots I haven't yet had the time to update.

/cc @fxn

These methods are nodoc so we should not document them.

This allows us to remove `Type::Value#klass`, as it was only used for
multi-parameter assignment to reach into the types internals. The
relevant type objects now accept a hash in addition to their previous
accepted arguments to `type_cast_from_user`. This required minor
modifications to the tests, since previously they were relying on the
fact that mulit-parameter assignement was reaching into the internals of
time zone aware attributes. In reaility, changing those properties at
runtime wouldn't change the accessor methods for all other forms of
assignment.

We added a comparison to "id", and call to `self.class.primary_key` a
*lot*. We also have performance hits from `&block` all over the place.
We skip the check in a new method, in order to avoid breaking the
behavior of `read_attribute`

Isolate access to @associations_cache and @aggregations cache to the Associations and Aggregations modules, respectively.

This includes replacing the `association_cache` accessor with a more
limited `association_cached?` accessor and making `clear_association_cache`
and `clear_aggregation_cache` private.

As we move towards removing the `klass` method from `column`, the else
clause as it stands right now will become the only case.

Minor refactoring of looping behavior for aggregation

At the point we want to add the reflection we already know the
reflection is of the AggregateReflection type so we can call a specific
method

Move model mutation to the methods that are called on the model.

particularly, `all(options)` would warn

This reverts commit 14fc8b34.

Reason: we need to discuss a better path from this removal.

Conflicts:
	activerecord/lib/active_record/reflection.rb
	activerecord/test/cases/base_test.rb
	activerecord/test/models/developer.rb

This feature adds a lot of complication to ActiveRecord for dubious
value. Let's talk about what it does currently:

class Customer < ActiveRecord::Base
  composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
end

Instead, you can do something like this:

    def balance
      @balance ||= Money.new(value, currency)
    end

    def balance=(balance)
      self[:value] = balance.value
      self[:currency] = balance.currency
      @balance = balance
    end

Since that's fairly easy code to write, and doesn't need anything
extra from the framework, if you use composed_of today, you'll
have to add accessors/mutators like that.

Closes #1436
Closes #2084
Closes #3807

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

Only constantize class_name once.

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

This makes it possible to filter invalid input values before they are passed
into the value-object (like empty strings). This behaviour is only relevant
if the :allow_nil options is set to true. Otherwise you will get
the resulting NoMethodError.

Fixed typo in composed_of example with Money#<=>, was comparing amount itself instead of other_money.amount

- persisted? is the API defined in ActiveModel
- makes it easier for extension libraries to conform to ActiveModel APIs
  without concern for whether the extended object is specifically
  ActiveRecord

[#5927 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>