[Sqlalchemy-tickets] Issue #3661: Add join and outerjoin classmethods to declarative classes (zzzeek/sqlalchemy)

[Haleemur A. <iss...@bi...>]

New issue 3661: Add join and outerjoin classmethods to declarative classes
https://bitbucket.org/zzzeek/sqlalchemy/issues/3661/add-join-and-outerjoin-classmethods-to

Haleemur Ali:

Given a database described via declarative syntax


```
#!python
from sqlalchemy import select, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)

class Address(Base):
    __tablename__ = 'addresses'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer)
    street = Column(String)
    city = Column(String)
    postcode = Column(String)
    region = Column(String)
```
The following query written in sqlalchemy expression language is not valid:

```
#!python
stmt = select(
    [User.name, Address.city]
).select_from(User.join(Address, User.id == Address.user_id))
```
It raises `AttributeError: type object 'User' has no attribute 'join'`.

However, the same expression written using `User.__table__` is valid, i.e.

```
#!python
>>>str(User.__table__.join(Address, User.id == Address.user_id))
'users JOIN addresses ON users.id = addresses.user_id'
```
produces the desired join expression.

The `join` & `outerjoin` functions imported from `sqlalchemy.sql.expression` accept declarative classes as input and is able to produce the desired output

```
#!python
>>>from sqlalchemy.sql.expression import join, outerjoin
>>>str(join(User, Address, User.id == Address.user_id))
'users JOIN addresses ON users.id = addresses.user_id'
>>>str(outerjoin(User, Address, User.id == Address.user_id))
'users LEFT OUTER JOIN addresses ON users.id = addresses.user_id'
```

In my opinion this is a simple fix to add `join` & `outerjoin` functions as `classmethods` to declarative classes through the mapper. That way both classically declared mappings and modern declarative mappings will be consistently usable in sqlalchemy expression language.

I'd be happy to submit a PR for this change.