The ProjectRole collection, sharded in the project databases as it is now, isn't useful for determining which projects a particular user has access to. Create a migration and/or a script to move ProjectRole into a unified collection in the main database.
This will also involve removing the 'projects' property and my_projects method on User projects, as these have been broken for a long time. my_projects() may be rewritten as a query on the (unified) ProjectRole collection.
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