When table has a composite primary key, the `primary_key` method for
sqlite3 and postgresql was only returning the first field of the key.

Ensures that it will return nil instead, as AR dont support composite pks.

Closes #7247.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/models/owner.rb

ActiveRecord::Base `save` and `save!` take an option boolean
`:touch` parameter since #18225 (stems from #18202).

This commit document that parameter.

[ci skip]

Fixes #18237

This is no longer required now that we are injecting a type caster
object into the Arel table, with the exception of uniqueness
validations. Since it calls `ConnectionAdapter#type_cast`, the value has
already been cast for the database. We don't want Arel to attempt to
cast it further, so we need to continue wrapping it in a quoted node.
This can potentially go away when this validator is refactored to make
better use of `where` or the predicate builder.

This API will require much less consuming code to change to accomodate
the removal of automatic type casting from Arel. As long as the
predicates are constructed using the `arel_table` off of an AR subclass,
there will be no changes that need to happen.

It is already docummented at persistence.rb.

[ci skip]

We only support classes which provide a no-args constructor to use as a
default value. We can provide a more helpful error message if we catch
this when `serialize` is called, rather than letting it error when you
try to assign the attribute.

Fixes #18224

Example:

    create_table :foos, id: :bigint do |t|
    end

/cc @sgrif

Most of the documentation very closely mirrors the matching
docs from `SchemaStatements`. I reduced duplicated copy and
added links to the underlying methods for the user to follow.

Because call the `column` method and set the `options[:primary_key]` is
handled at `super`, here need only treat the `options[:default]`.

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of a larger refactoring to remove type casting behavior from Arel

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of the larger refactoring to remove type casting from Arel.

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

Part of the larger refactoring to remove type casting from Arel. We can
inform it that we already have the right type by wrapping the value in
an `Arel::Nodes::Quoted`. This commit can be reverted when we have
removed type casting from Arel in Rail 5.1

There are several valid cases where right now we can't determine the
association's class in a call to `where`. In these cases, we can fall
back to casting by looking up the column from the connection adapter
(which is what happens right now when we fall through to Arel)

This is ugly, and since we're trying to separate the concept of a type
from a column, I'd like to remove it in the future. The problem
basically comes down to this:

    Liquid.joins(molecules: :electrons)
      .where("molecules.name" => "something", "electrons.name" => "something")

The hash in this case will turn into:

    {
      molecules: { name: "something" },
      electrons: { name: "something" },
    }

What we actually need is:

    {
      molecules: {
        name: "something",
        electrons: { name: "something" },
      }
    }

/cc @mrgilman

This code could use some much heavier refactoring. It looks like
`build_relation` duplicates most of the logic of `Relation#where` and
`PredicateBuilder` with regards to handling associations and attribute
aliases

Part of the larger refactoring to remove type casting from Arel. Since
we've already cast the value a few lines above, we don't need to re-cast
it later. We can inform Arel of this by wrapping it in an
`Arel::Nodes::Quoted`, which will no longer be required in Rails 5.1

We will always have the correct type for this query, so no casting is
needed. We inform Arel that we already have the right type by wrapping
it in an `Arel::Nodes::Quoted` (which we will no longer need to do in
Rails 5.1)

This will allow eager type casting to take place as needed. There
doesn't seem to be any particular reason that the `in` statement was
forced for single values, and the commit message where it was introduced
gives no context.

See
https://github.com/rails/rails/commit/d90b4e2615e8048fdeffc6dffe3246704adee01f

Part of a larger refactoring to remove type casting from Arel.

/cc @mrgilman

[Sean Griffin & Melanie Gilman]

A custom object is required for this, as you cannot build a range
object out of `Arel::Nodes::Quoted` objects. Depends on the changes
introduced in
https://github.com/rails/arel/commit/cf03bd45e39def057a2f63e42a3391b7d750dece

/cc @mrgilman