Include Expression could not handle extra join

[Condra963]

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  Summary of the changes
  If you look into following issues, Expressions: Create Navigation Property #12965 and Should we be treating objects created by the user in a query differently from the ones we materialize? #12668, you can see i want to create an extra join on
  properties that i marked for it. Everytime i do a query i want to Coalesce these column with a
  Joined Table. When i did this i ran into multiple issues, as example when you have a Navigation
  Property it was always filled in, EFCore saw this as an object when you use Expression.New and not
  anymore as an EntityType. The reason for this is explained in the issues and that's because they
  make use of Tracking.
  As you can see in the issue Should we be treating objects created by the user in a query differently from the ones we materialize? #12668 i asked if it is a good idea to keep creating an EntityType if
  AsNoTracking Expression is added. While i am waiting i created a work around in my code that make
  this work on EFCore.Relational level, i made an overwrite of the SelectExpression class. While i was
  creating this work around i bumped into a problem and that problem is when i add an Include
  Expression EFCore first creates a SelectExpression and than convert it to an InnerJoin. My
  modification adds to the SelectExpression of the Include Expression also a LeftOuterJoin. Because
  of this the innerSelectExpression had 2 tables instead of 1. So what i did is use the First table for the
  InnerJoin and add my LeftOuterJoin also as a Table to the outerSelectExpression.

  This solved my issue, so now i hope this could be implemented in EFCore so i can keep working with
  this final work around until there comes a better solution.

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[smitpatel]

This will not work in all scenarios.
TryFlattenJoin requires both outer & inner sequence translation to be in the form of select expression without any additional tables. If there are multiple tables then the select expression needs to be pushed down prior to coming here. That is the main reason for using Single().
Adding additional tables may have worked for you but it is breaking a huge invariant in current system and can break a lot of things.
In the light of #12795 , We should not take this PR.

[Condra963]

Could you give me some scenario('s) where this should not work please? Then i am able to create some unit tests and make this work properly. Because i really need a way to work this out... In the mean while i try to recognize the Include Expression and try to figure out a way to do my second join after the inner join.

Thanks for the quick feedback.

[ajcvickers]

@Condra963 As @smitpatel says, we are reluctant to take this PR, but in order for us to make a better determination can you please add a test that demonstrates how the code change is useful?

[Condra963]

For now i worked arround it, when i find some time i will add one or more test cases how this is useful.