diffs cardinality many through, inc tests

[leftstanding]

Resolves polymorphism association many through causes error.
Does this by first checking if the association has already been diffed in the association
If not then builds the diff.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (b91baf0) 85.71% compared to head (f2f813d) 87.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   85.71%   87.25%   +1.54%     
==========================================
  Files           1        1              
  Lines          91      102      +11     
==========================================
+ Hits           78       89      +11     
  Misses         13       13              

Files	Coverage Δ
lib/ecto_diff.ex	87.25% <100.00%> (+1.54%)	⬆️

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[luizmiranda7]

ℹ️ I don't quite understand what is expected value here... perhaps an empty list?

[leftstanding]

If the through key is present, but the assoc_field key is not, the return could be [nil], so _ -> was to handle both [] || [nil]. I have changed how we handle this so it can expect only empty lists.

[dimakula]

ℹ️ This should just be an empty list:

Suggested change

	_ ->
	[] ->

[leftstanding]

At the time, the return could also be [nil]. Changed the way we handle this.

[brunohkbx]

⚠️ Also missing a CHANGELOG entry

[leftstanding]

Unfortunately, without the select_fields option changes, testing the fallback when getting a diff for cardinality many through is difficult. In a sense, this is good; almost always EctoDiff should be able to retrieve the diff for the association via the through association diff, eliminating duplicate work being required. However, this has been tested locally by disabling the default path (get already diffed values) and verifying that all the tests still pass. Currently, the only test for this case is when there is no change on the through association, which triggers the fallback and as a side effect verifies that the default path matches the fallback path.