Save has_one associations only if record has changes

Prevents save related callbacks such as `after_commit` being
triggered when `has_one` objects are already persisted and have no
changes.
Handle missing environment from non empty config

If using a `DATABASE_URL` and a `database.yml`. The connection information in `DATABASE_URL` should be merged into whatever environment we are in. As released in 4.1.0rc1 if someone has a database.yml but is missing a key like production:

```yml
development:
  host: localhost

```

Then the check for blank config will return false so the information from the `DATABASE_URL` will not be used when attempting to connect to the `production` database and the connection will incorrectly fail.

This commit fixes this problem and adds a test for the behavior.

In addition the ability to specify a connection url in a `database.yml` like this:

```
production: postgres://localhost/foo
```

Was introduced in 4.1.0rc1 though should not be used, instead using a url sub key

```
production:
  url: postgres://localhost/foo
```

This url sub key was also introduced in 4.1.0rc1 though the `production: postgres://localhost/foo` was not removed. As a result we should not test this behavior.
Distinguish ConnectionNotEstablished messages: no conn pool for the class, or no conn available from the pool
typos fixes
Fixed typo in comment about MAX_ID.
Merge pull request #14164 from vajrasky/fix_typo_max_id

Fixed typo in comment about MAX_ID.
Tweak Configuring Guide language

the fact that Rails is multi-thread by default is not very relevant. see https://github.com/rails/rails/pull/13998 [ci skip]
Add warning when user tried to create model with pluralize name.
  1. Generate model with correct_name.
  2. It will help new users to avoid mistakes when tried to create model with wrong name.
Merge pull request #13515 from kuldeepaggarwal/f-model-generation

Added warning when user tried to create model with pluralize name.
:scissors:

This commit also addresses rails/docrails#169 and rails/rails#14159
Add tests for default scope behaviour change

See #13875
dynamically define PostgreSQL OID range types.

This gets AR working with custom defined range types. It also
removes the need for subtype specific branches in `OID::Range`.

This expands the interface of all `OID` types with the `infinity` method.
It's responsible to provide a value for positive and negative infinity.
deprecate support for pg ranges with excluding beginnings.

The Ruby Range object does not support excluding beginnings.
We currently support excluding beginnings for some subtypes using
manually by incrementing them (now using the `#succ` method).
This is approach is flawed as it's not equal to an excluding beginning.

This commit deprecates the current support for excluding beginnings.
It also raises an `ArgumentError` for subtypes that do not implement the `succ`
method.

This is a temporary solution to get rid of the broken state. We might still
add complete support for excluding beginnings afterwards. (Probably with a
new `PGRange` object, which acts like a `Range` but has excluding beginnings.
update version to 4.2.0.alpha
Merge pull request #13793 from senny/postgres/dynamic_range_oid

Dynamically define PostgreSQL Range OIDs
Coerce strings when reading attributes.
Fixed STI classes not defining an attribute method if there is a
conflicting private method defined on its ancestors.

The problem is that `method_defined_within?(name, klass, superklass)`
only works correclty when `klass` and `superklass` are both `Class`es.

If both `klass` and `superklass` are both `Class`es, they share the
same inheritance chain, so if a method is defined on `klass` but not
`superklass`, this method must be introduced at some point between
`klass` and `superklass`.

This does not work when `superklass` is a `Module`. A `Module`'s
inheritance chain contains just itself. So if a method is defined on
`klass` but not on `superklass`, the method could still be defined
somewhere upstream, e.g. in `Object`.

This fix works by avoiding calling `method_defined_within?` with a
module while still fufilling the requirement (checking that the
method is defined withing `superclass` but not is not a generated
attribute method).

4d8ee288 is likely an attempted partial fix for this problem. This
unrolls that fix and properly check the `superclass` as intended.

Fixes #11569.
Introduce Concern#class_methods and Kernel#concern
Extract local cache middleware

Extract LocalCache Middleware, so it can requires rack dependencies,
without adding rack dependencies to `AS::Cache::Strategy::LocalCache`.
Group assets options in production env template