"Cannot update 'companion_ids' and 'companion_ids' at the same time"

[simonv3]

A Moped::Errors::OperationFailure occurred in crops#update:

 The operation: #<Moped::Protocol::Command
 @length=75
 @request_id=1198
 @response_to=0
 @op_code=2004
 @flags=[]
 @full_collection_name="app28600065.$cmd"
 @skip=0
 @limit=-1
 @selector={:getlasterror=>1, :w=>1}
 @fields=nil>
failed with error 16837: "Cannot update 'companion_ids' and 'companion_ids' at the same time"

See https://github.com/mongodb/mongo/blob/master/docs/errors.md
for details about this error.
 app/models/crop.rb:120:in `block in backlink_companion_crops'