Using date_part on an aliased field generates new alias
I'm confused...what's different in the 2nd and 3rd queries? I can see that in the first it uses t3 for the alias, but don't understand why it uses t4 in the last query.
 Oops, I see: it's using Person in the second example query. I swear I looked at them for a minute and didn't see that. Ok, makes sense.
 d7720d95335d607add81f2d2dbc5e8fc5424b1fd

This is a stab at an implementation. What do you think? I just tested the example and tested it in my project and I can now remove a lot of work-arounds! Keep up the awesome work! This is fixed in 3.0a, commit: f81a30b621bebef103950a2667aed6df2bf69ddd
When using the `date_part` functionality (and others, e.g. `contains_any` in ArrayField), the field is passed as `self`. This works beautifully if you are using the model it is defined in, but, when using it on an aliased table, it generates a new alias.

Below is a very crude example from how I found this "problem". This first query shows that the part of the query that will be constructed works when called with the aliased field: `fn.DATE_PART('year', Client.birth) > 1980)`.

Calling `Client.birth.year` is what is generating the problem here, as it will refer to the `Person` model instead of the `Client` alias, generating the fourth alias. Would I not alias `Owner`, but use the `Person` model, it would construct the where part based on that alias, instead of the `Client`.

```
from peewee import *
import logging


database = SqliteDatabase(':memory:')

logger = logging.getLogger('peewee')
logger.setLevel(logging.DEBUG)
logger.addHandler(logging.StreamHandler())


class Base(Model):
    class Meta:
        database = database


class Person(Base):
    name = CharField()
    birth = DateField()


class Traject(Base):
    owner = ForeignKeyField(Person, related_name='trajects_owned')
    client = ForeignKeyField(Person, related_name='trajects')


Person.create_table()
Traject.create_table()

john = Person.create(name='John', birth='1979-1-1')
suzi = Person.create(name='Suzi', birth='1982-1-1')
louise = Person.create(name='Louise', birth='1970-1-1')
cody = Person.create(name='Cody', birth='1990-1-1')

Traject.create(owner=john, client=suzi)
Traject.create(owner=john, client=louise)
Traject.create(owner=john, client=cody)

Client = Person.alias()
Owner = Person.alias()

trajects = (Traject.select()
            .join(Client, on=(Traject.client == Client.id))
            .join(Owner, on=(Traject.owner == Owner.id))
            .where(fn.DATE_PART('year', Client.birth) > 1980))

assert trajects.count() == 2
# good so far

trajects = (Traject.select()
            .join(Client, on=(Traject.client == Client.id))
            .join(Person, on=(Traject.owner == Person.id))
            .where(Client.birth.year > 1980))

assert trajects.count() == 0
# this should be two, but the where is based on the owner

trajects = (Traject.select()
            .join(Client, on=(Traject.client == Client.id))
            .join(Owner, on=(Traject.owner == Owner.id))
            .where(Client.birth.year > 1980))

assert trajects.count() == 2
# this will generate an exception
```

Here are the two generated queries by _peewee_.

```
SELECT Count(*)
FROM "traject" AS t1
INNER JOIN "person" AS t2 ON ("t1"."client_id" = "t2"."id")
INNER JOIN "person" AS t3 ON ("t1"."owner_id" = "t3"."id")
WHERE (DATE_PART(?, "t2"."birth") > ?)
-- parameters: ['year', 1980]

SELECT Count(*)
FROM "traject" AS t1
INNER JOIN "person" AS t2 ON ("t1"."client_id" = "t2"."id")
INNER JOIN "person" AS t3 ON ("t1"."owner_id" = "t3"."id")
WHERE (date_part(?, "t3"."birth") > ?)
-- parameters: ['year', 1980]

SELECT Count(*) FROM "traject" AS t1
INNER JOIN "person" AS t2 ON ("t1"."client_id" = "t2"."id")
INNER JOIN "person" AS t3 ON ("t1"."owner_id" = "t3"."id")
WHERE (date_part(?, "t4"."birth") > ?)
-- parameters: ['year', 1980])
```

The exception that was caused by the third query:

```
Traceback (most recent call last):
  File "app.py", line 61, in <module>
    assert trajects.count() == 2
  File "$/site-packages/peewee.py", line 2865, in count
    return self.aggregate(convert=False) or 0
  File "$/site-packages/peewee.py", line 2858, in aggregate
    return self._aggregate(aggregation).scalar(convert=convert)
  File "$/site-packages/peewee.py", line 2637, in scalar
    row = self._execute().fetchone()
  File "$/site-packages/peewee.py", line 2628, in _execute
    return self.database.execute_sql(sql, params, self.require_commit)
  File "$/site-packages/peewee.py", line 3461, in execute_sql
    self.commit()
  File "$/site-packages/peewee.py", line 3285, in __exit__
    reraise(new_type, new_type(*exc_args), traceback)
  File "$/site-packages/peewee.py", line 3454, in execute_sql
    cursor.execute(sql, params or ())
peewee.OperationalError: no such column: t4.birth
```
