Implement lazy-loading of navigation properties

[ajcvickers]

This issue tracks work on lazy-loading of relationships in EF Core. Subscribe to this issue to get updates when progress is made on lazy-loading. This issue is locked for now to reduce noise. Please use issue #3797 for general discussion on lazy-loading in EF Core.

[ajcvickers]

Just to clarify the status on this:

• We do intend to implement lazy loading in EF Core.
• There are issues with dynamic proxies and things have moved on considerably since EF4. We ultimately want to instead use Roslyn to re-write assemblies to add appropriate hooks as part of build. This is quite a lot of work, but we feel it will deliver the best experience in the long term.
• In the short term (hopefully 2.1) we plan to make it easy to configure lazy loading, but it will require some changes to the entity classes to make it work--i.e. it won't be true POCO until the Roslyn work is done.
• We will try to do all of this using appropriate hooks so that other third-party libraries could implement other types of AOT injections, either through Roslyn or proxies. If you try to do this, but hit roadblocks, then let us know since we may be able to do smaller things to unblock you, or you may be able to submit PRs.

[ajcvickers]

Initial lazy-loading PR for 2.1 has been merged: #10624

[ajcvickers]

Initial PR for of a lazy-loading proxies package: #10720

[ajcvickers]

Note for triage: work still to do:

• Configuration of which entities/properties can be lazy loaded. (As of now it's all or nothing.)
• Roslyn-based solution
• Lazy-loading and no-tracking entities
• Disconnect the lazy-loader on detach, connect on attach
• Allow property injection
• Configuration of the constructor/property binding

[ajcvickers]

Triage notes: for 2.1 we will try to do:

• Lazy-loading and no-tracking entities
• Disconnect the lazy-loader on detach, connect on attach
• Allow property injection

Other items will be post 2.1 and are tracked by #10787 #10788 #10789

[ajcvickers]

Notes from triage:

• Async infrastructure added
• Punting lazy-loading for no-tracking entities: Support lazy and explicit loading from entities queried with NoTracking behavior #10042