Allow keys and indexes to use properties on non-collection complex types

[AndriySvyryd]

Split off from #13947

Depends on #31236

Related to #11336

[ajcvickers]

This issue is in the Backlog milestone. This means that it is not planned for the next release (EF Core 9.0). We will re-assess the backlog following the this release and consider this item at that time. However, keep in mind that there are many other high priority features with which it will be competing for resources. Make sure to vote (👍) for this issue if it is important to you.

[nev-21]

Hello! This feature is really important for us.

[sjb-sjb]

This would be really useful. Complex Types are needed for hiding functionality and managing the relationship of fields in the database.

For example say I have labels in my data, and some but not all of these labels have related data. I would like to use a complex type to manage the interaction of the text label vs the referenced data. If I cannot index the text label then it is impossible to perform the required operations.

In my particular situation I don’t need a composite key, just a key on one property within the complex type.