Simplify

[blag]

Using some of the code from Django, we can greatly simplify and even remove some of our code.

Description

• I adjusted our middleware to inherit from Django's so we don't have to duplicate any of that code
  • Django imports the session engine only once, instead of on each request - this also simplifies our module imports
  • We don't have to define our own process_response at all, since it wasn't materially different from Django's
• I tweaked our session backend to inherit from Django's, and tweaked it to use some of the methods defined in the superclass
  • Redefine get_model_class to import and use our Session model, which allows other methods (including ones from the superclass) to use self.model
  • Use self.model instead of our own Session model directly
  • Remove our own exists method entirely (which will use Django's)
  • Define and override _get_session_from_db, so we can remove the load method
  • Override and use Django's create method
  • Define and override create_model_instance so we don't have to define our own save method
  • Remove our own copied version of the delete and clear_expired methods

Motivation and Context

We duplicated a lot of code from django.contrib.sessions. The less code we duplicate, the less code we have to maintain ourselves.

This might also make it easier to add a cached_db backend similar to Django's.

How Has This Been Tested?

Normal tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Reverse-compatible code improvement

Checklist:

• My code follows the code style of this project.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #175 (ef28ea0) into master (5ab2c57) will increase coverage by 1.92%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   94.92%   96.84%   +1.92%     
==========================================
  Files          16       16              
  Lines         768      698      -70     
  Branches       65       56       -9     
==========================================
- Hits          729      676      -53     
+ Misses         29       16      -13     
+ Partials       10        6       -4     

Files	Coverage Δ
user_sessions/backends/db.py	100.00% <100.00%> (+8.82%)	⬆️
user_sessions/middleware.py	100.00% <100.00%> (+29.72%)	⬆️

... and 2 files with indirect coverage changes

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[WhyNotHugo]

I have some questions of some bits that are not clear.

4527600 looks good, I can go ahead and merge that single commit in the meantime.

[WhyNotHugo]

Oh, darn, I can't merge 4527600 since merging into this repository is only allowed by creating a separate PR.

I'll merge them all together when all patches have are ready.

[blag]

I have responded to your questions. If I have done so to your satisfaction, please resolve them. Otherwise I'm happy to answer additional questions or explain what's going on in a bit more detail. The interplay in the middleware and backends between Django's superclasses and ours can be a little complicated, so please take your time and ask for whatever explanations you need.

I realize that this PR is quite a big change and while I'd like to see it merged, I'm trying very hard to not be pushy about it. 😄

There is an argument to be made that our previous code was much more easily understandable than the current code, even though it duplicated a lot of code from Django. However, since it doesn't seem like this project has tons of attention paid to it, reducing the amount of code we have to maintain in this project and relying on subclassing Django objects is a worthwhile tradeoff in my opinion, even if it does make the code slightly more difficult to figure out what it's doing.

[blag]

I added a few more comments to give some pointers as to where certain methods are used by methods on the superclass. Happy to add more if they'll be useful or helpful.