Use simple entity type for simple join tables

[ajcvickers]

A Dictionary is overkill for a type that will only store two key properties, which is the common case for many-to-many with non-composite keys on either end. Instead, consider using:

public class JoinEntity<TLeft, TRight>
{
    public virtual TLeft LeftId { get; set; }
    public virtual TRight RightId { get; set; }
}

[roji]

Another option is to simply use ValueTuple.

Note that there's a perf impact here as well, since Dictionary is much slower.

[ajcvickers]

Note from triage: punt on this for now, but:

• Document that the implicitly chosen type may change in future releases. Document that implicitly chosen type for join table may change EntityFramework.Docs#2563
• Provide a way for the default to be set so that people can get back to the current type if they do depend on it.

[willnationsdev]

Just out of curiosity, how is this proposal different from EntityTypeBuilder<TRightEntity> UsingEntity<TJoinEntity>(...)? I have scenarios where I map a simple entity to a many-to-many join table and that is already supported. I do, however, have to manually specify the left & right sides of the mapping entity's relationships, so is this about making that process automatic for Plain Old Data types?

[ajcvickers]

@willnationsdev This is about changing the default.