For example, if you had the following class:
```
class Person(Document):
name = StringField()
age = IntField()
```
You could instantiate an object of such class by doing one of the following:
1. `new_person = Person('Tom', 30)`
2. `new_person = Person('Tom', age=30)`
3. `new_person = Person(name='Tom', age=30)`
From now on, only option (3) is allowed.
Supporting positional arguments may sound like a reasonable idea in this
heavily simplified example, but in real life it's almost never what you want
(especially if you use inheritance in your document definitions) and it may
lead to ugly bugs. We should not rely on the *order* of fields to match a given
value to a given name.
This also helps us simplify the code e.g. by dropping the confusing (and
undocumented) `BaseDocument._auto_id_field` attribute.
This closes#2096. Previous documentation of the shard_key meta attribute was
missing the crucial point that it really only matters if your collection is
sharded over a compound index.
Previously, we were running the test suite for several combinations of MongoDB,
Python, and PyMongo:
- PyPy, MongoDB v2.6, PyMongo v3.x (which really means v3.6.1 at the moment)
- Python v2.7, MongoDB v2.6, PyMongo v3.x
- Python v3.5, MongoDB v2.6, PyMongo v3.x
- Python v3.6, MongoDB v2.6, PyMongo v3.x
- Python v2.7, MongoDB v3.0, PyMongo v3.5.0
- Python v3.6, MongoDB v3.0, PyMongo v3.5.0
- Python v3.5, MongoDB v3.2, PyMongo v3.x
- Python v3.6, MongoDB v3.2, PyMongo v3.x
- Python v3.6, MongoDB v3.4, PyMongo v3.x
- Python v3.6, MongoDB v3.6, PyMongo v3.x
There were a couple issues with this setup:
1. MongoDB v2.6 – v3.2 have reached their End of Life already (v2.6 almost 3
years ago!). See the "MongoDB Server" section on
https://www.mongodb.com/support-policy.
2. We were only testing two recent-ish PyMongo versions (v3.5.0 & v3.6.1).
We were not testing the oldest actively supported MongoDB/PyMongo/Python
setup.
This PR updates the test matrix so that these problems are solved. For the
sake of simplicity, it does not yet attempt to cover MongoDB v4.0:
- PyPy, MongoDB v3.4, PyMongo v3.x (aka v3.6.1 at the moment)
- Python v2.7, MongoDB v3.4, PyMongo v3.x
- Python v3.5, MongoDB v3.4, PyMongo v3.x
- Python v3.6, MongoDB v3.4, PyMongo v3.x
- Python v2.7, MongoDB v3.4, PyMongo v3.4
- Python v3.6, MongoDB v3.6, PyMongo v3.x
