railsadminteam / rails_admin / 37282845852

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Resolve the inverse of a polymorphic association

`has_one :comment, as: :commentable` states that the inverse
association on the child is named :commentable, but neither adapter
reported it: both read only an explicit :inverse_of option, which such
a declaration never has. (ActiveRecord's own reflection does resolve
it; the adapter was throwing that away. Mongoid does not resolve it at
all, so `as` is the only source available to both - which makes the
adapter the right place to normalize this.)

Because of that, the nested form for a polymorphic has_one rendered
the child's back-reference to its own parent, submitting a blank
commentable_type/commentable_id pair. ActiveRecord quietly tolerates
that, and so did Mongoid up to 8.x, so the bug went unnoticed under
both. Mongoid 9 rejects it outright with
Mongoid::Errors::UnrecognizedModelAlias ('Cannot find any model with
type ""'), which is how it finally surfaced.

nested_field_association? learns nothing new - it already compared
field.name against nested_in.inverse_of - but that comparison sat
behind a guard requiring the *child* to declare inverse_of, which a
polymorphic belongs_to cannot do (its inverse is genuinely ambiguous).
Promote it to a branch of its own.

The nested-many case has covered this shape since 'hides fields that
are deeply nested with inverse_of'; add the missing polymorphic
counterpart, which fails under both ORMs without the fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
(cherry picked from commit 91e9cca88)

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