MONGOID-5785 allow named scopes to remove a default scope

[jamis]

As currently implemented, named scopes are merged with the default scope (see here). This means you cannot declare a named scope that unsets any part of the default scope; if you try, the default scope will simply be merged back into it.

Merging the default scope is not necessary, because any scope you pass in will already be based on the default scope:

class Person
  include Mongoid::Document

  default_scope { where(deleted_at: nil) }
  scope :germans, -> { where(country: 'DE') }
end

Here, Person.germans will always use the default scope, even without it being merged in, because calling where will initialize a new Criteria object using that default scope. Merging the default scope is redundant.

However, because this has been this way for a very long time (basically since the feature was first introduced prior to 2014), we risk breaking code that inadvertently depends on this behavior. Thus, I've opted to put the fix behind a flag, which we can flip when we eventually release Mongoid 10.

[johnnyshields]

@jamis @comandeo-mongo please roll this back. This behavior should stay as-is with no feature flag or intention to change it.

The .unscoped method works perfectly in this case today. It is completely counter-intuitive that using a named scope would magically cause the default scope to disappear.

Further example of how this will break apps in all sorts of fun and exciting ways here:

https://jira.mongodb.org/browse/MONGOID-5785?focusedCommentId=6461746&focusedId=6461746&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-6461746

[johnnyshields]

Correction: I misunderstood the change here. Since it's only calling ".unscoped" within a named scope it should be fine.

[stanley90]

Hi, I'm a bit late to the party (was on vacation), just want to pinpoint that using unscoped does not solve the problem I described, because it removes everything, whereas I only want to remove a specific part of the default scope (paranoia, precisely). I'll take a closer look at this when I'm back at work. Thanks

[jamis]

@stanley90 -- thanks for chiming in. The PR includes tests that confirm correct behavior using unscoped, as well as with remove_scoping (see https://github.com/mongodb/mongoid/pull/5832/files#diff-d374f75265fac28314512810734ae94770db90759b5b05d5f76e546e01acecfaR1258-R1259). I'm confident it works correctly in both cases.

[stanley90]

@jamis I patched the Scopable.define_scope_method in my project and confirm it does solve the problem. I'll keep the patch until I'm able to upgrade Mongoid 😀 Thanks