Happy 2021 Mongoid team! I found another bug over the holidays.

Suppose I have this class:

class Customer
  embeds_one :home_address, class_name: 'Address', as: :addressable
  embeds_one :work_address, class_name: 'Address', as: :addressable
end

class Address
  embedded_in :addressable, polymorphic: true
end

In my app, strange things happen when I do:

customer.work_address = Address.new

I see cases of this assigning to home_address, which is very scary!

I don't fully understand what's going on, but it appears that Mongoid gets tripped up assigning an "embedded_in" relationship in this case as there is ambiguity here.

Reproduction:

**I've reproduced this issue here: https://github.com/mongodb/mongoid/pull/4939

As mentioned above, I see this issue in my app when doing `customer.work_address = Address.new`. However, so far I've only been able to reproduce it with:

customer.work_address.addressable = customer

If you study how the binding code works, this seems like a sufficient reproduction to get started.

Proposed Behavior:

Attempting to assign address.addressable = customer should throw an "Ambiguous assignment error" in this case. We should only be allowed to assign customer.home/work_address = address; not the other way around.