Post.where('id = 1').or(Post.where('id = 2'))
    # => SELECT * FROM posts WHERE (id = 1) OR (id = 2)

[Matthew Draper & Gael Muller]

`bound_attributes` is now used universally across the board, removing
the need for the conversion layer. These changes are mostly mechanical,
with the exception of the log subscriber. Additional, we had to
implement `hash` on the attribute objects, so they could be used as a
key for query caching.

The column is primarily used for type casting, which we're trying to
separate from the idea of a column. Since what we really need is the
combination of a name, type, and value, let's use the object that we
already have to represent that concept, rather than this tuple. No
consumers of the bind values have been changed, only the producers
(outside of tests which care too much about internals). This is
*finally* possible since the bind values are now produced from a
reasonable number of lcoations.

Attempting to grok this code by refactoring it as I go through it.

The bind values can come from four places. `having`, `where`, `joins`,
and `from` when selecting from a subquery that contains binds. These
need to be kept in a specific order, since the clauses will always
appear in that order. Up until recently, they were not.

Additionally, `joins` actually did keep its bind values in a separate
location (presumably because it's the only case that people noticed was
broken). However, this meant that anything accessing just `bind_values`
was broken (which most places were). This is no longer possible, there
is only a single way to access the bind values, and it includes joins in
the proper location. The setter was removed yesterday, so breaking `+=`
cases is not possible.

I'm still not happy that `joins` is putting it's bind values on the
Arel AST, and I'm planning on refactoring it further, but this removes a
ton of bug cases.

Contrary to my previous commit message, it wasn't overkill, and led to
much cleaner code.

[Sean Griffin & anthonynavarre]

The last place that was assigning it was when `from` is called with a
relation to use as a subquery. The implementation was actually
completely broken, and would break if you called `from` more than once,
or if you called it on a relation, which also had its own join clause,
as the bind values would get completely scrambled. The simplest solution
was to just move it into its own array, since creating a `FromClause`
class for this would be overkill.

All of its uses have been moved to better places

This fixed an issue where `having` can only be called after the last
call to `where`, because it messes with the same `bind_values` array.
With this change, the two can be called as many times as needed, in any
order, and the final query will be correct. However, once something
assigns `bind_values`, that stops. This is because we have to move all
of the bind values from the having clause over to the where clause since
we can't differentiate the two, and assignment was likely in the form
of:

`relation.bind_values += other.bind_values`

This will go away once we remove all places that are assigning
`bind_values`, which is next on the list.

While this fixes a bug that was present in at least 4.2 (more likely
present going back as far as 3.0, becoming more likely in 4.1 and later
as we switched to prepared statements in more cases), I don't think this
can be easily backported. The internal changes to `Relation` are
non-trivial, anything that involves modifying the `bind_values` array
would need to change, and I'm not confident that we have sufficient test
coverage of all of those locations (when `having` was called with a hash
that could generate bind values).

[Sean Griffin & anthonynavarre]

We've now removed all uses of them across the board. All logic lives on
`WhereClause`.

The code assumes that non-single-value methods mean multi value methods.
That is not the case. We need to change the accessor name, and only
assign an array for multi value methods

We're still using it in `where_unscoping`, which will require moving
additional logic.

Yes, I know, I called it a factory so I'm basically the worst person
ever who loves Java and worships the Gang of Four.

The way that bind values are currently stored on Relation is a mess.
They can come from `having`, `where`, or `join`. I'm almost certain that
`having` is actually broken, and calling `where` followed by `having`
followed by `where` will completely scramble the binds.

Joins don't actually add the bind parameters to the relation itself, but
instead add it onto an accessor on the arel AST which is undocumented,
and unused in Arel itself. This means that the bind values must always
be accessed as `relation.arel.bind_values + relation.bind_values`.
Anything that doesn't is likely broken (and tons of bugs have come up
for exactly that reason)

The result is that everything dealing with `Relation` instances has to
know far too much about the internals. The binds are split, combined,
and re-stored in non-obvious ways that makes it difficult to change
anything about the internal representation of `bind_values`, and is
extremely prone to bugs.

So the goal is to move a lot of logic off of `Relation`, and into
separate objects. This is not the same as what is currently done with
`JoinDependency`, as `Relation` knows far too much about its internals,
and vice versa. Instead these objects need to be black boxes that can
have their implementations swapped easily.

The end result will be two classes, `WhereClause` and `JoinClause`
(`having` will just re-use `WhereClause`), and there will be a single
method to access the bind values of a `Relation` which will be
implemented as

```
join_clause.binds + where_clause.binds + having_clause.binds
```

This is the first step towards that refactoring, with the internal
representation of where changed, and an intermediate representation of
`where_values` and `bind_values` to let the refactoring take small
steps. These will be removed shortly.

This will allow all types which require no additional handling to use
prepared statements. Specifically, this will allow for `true`, `false`,
`Date`, `Time`, and any custom PG type to use prepared statements. This
also revealed another source of nil columns in bind params, and an
inconsistency in their use.

The specific inconsistency comes from a nested query coming from a
through association, where one of the inversed associations is not
bi-directional.

The stop-gap is to simply construct the column at the site it is being
used. This should simply go away on its own once we use `Attribute` to
represent them instead, since we already have all of the information we
need.

This is to help facilitate future refactorings, as the internal
representation is changed. I'm planning on having `where_values` return
an array that's computed on call, which means that mutation will have no
affect. This is the only remaining place that was mutating (tested by
replacing the method with calling `dup`)

With the old implementation, the bind values were created, and then we
search the attributes for `Relation` objects, and merge them. This
completely ignores the order that the actual `where` clause will use. If
all non-relation where parameters are before the relations, it will
work. However, if we query on both a relation and a value, with the
value coming second, it breaks. The order of the hash should not affect
the final query (especially since hashes being ordered is an
implementation detail)

I'm looking to introduce a `WhereClause` class to handle most of this
logic, and this method will eventually move over to there. However, this
intermediate refactoring should make that easier to do.

Looking through the blame, this logic used to be when we actually
created the bind tuple. My guess is that `nil` couldn't be handled there
at that time. It can, now.

In order to better facilitate refactoring, most places that mutated
`bind_values` have already been removed. One last spot snuck through.
Since we're no longer mutating the array, it also does not need to be
duped in `initialize_copy`.

This is cropping up all over the place. After a brief dive, I'm really
not sure why we have `arel.bind_values` at all. A cursory grep didn't
reveal where they're actually being assigned (it's definitely in AR, not
in Arel). I'd like to dig further into it, as I'm fairly certain we
don't actually need it, we just need a way for the predicate builder to
communicate merged binds upstream.

Fixes #18414

This adresses https://github.com/rails/rails/commit/1b7aa62b184c4410c99208f71b59bbac5c5f03be#commitcomment-9147803

This commit fixes the following case.

    User.where(User.arel_table[:created_at].lteq(1.year.ago)).unscope(where :created_at)

`where_sql` now requires that we pass it an engine. None of the manager
classes take an engine in their constructor.

We never actually make use of it on the table, since we're constructing
the select manager manually. It looks like if we ever actually were
grabbing it from the table, we're grossly misusing it since it's meant
to vary by AR class.

Its existence on `Arel::Table` appears to be purely for convenience
methods that are never used outside of tests. However, in production
code it just complicates construction of the tables on the rails side,
and the plan is to remove it from `Arel::Table` entirely. I'm not
convinced it needs to live on `SelectManager`, etc either.

on the joined assoiciation

Arel handles this for us automatically. Updated tests, as BindParam is
no longer a subclass of SqlLiteral. We should remove the second argument
to substitute_at entirely, as it's no longer used

We need to re-order the bind parameters since the AST returned by the
relation will have the where statement as the first bp, which breaks on
PG.

In practical terms, this allows serialized columns and tz aware columns
to be used in wheres that go through joins, where they previously would
not behave correctly. Internally, this removes 1/3 of the cases where we
rely on Arel to perform type casting for us.

There were two non-obvious changes required for this. `update_all` on
relation was merging its bind values with arel's in the wrong order.
Additionally, through associations were assuming there would be no bind
parameters in the preloader (presumably because the where would always
be part of a join)

[Melanie Gilman & Sean Griffin]