I am running on the master branch with rails 4. I found that the update_attributes always empties the associations' ids array

I set up two models, Entry and EntryAlias, with HABTM relation. The Entry model can create multiple EntryAlias through nested form.

accepts_nested_attributes_for :entry_aliases, :allow_destroy => true

Here is the parameters passing to the @entry.update_attributes(entry_params).

## entry_params
{"content"=>" ", "open"=>"1", "entry_aliases_attributes"=>{"0"=>{"id"=>"51db0cf181e0e7585f000007", "name"=>"test", "_destroy"=>"false"}}}

After updating @entry, the entry_ids array in the origin EntryAlias is emptied

## Original entry_alias 
{
    _id: ObjectId("51dafe4481e0e7f989000005"),
    entry_ids: [
        ObjectId("51db013481e0e73d46000003")
    ],
    name: "test"
}

## entry_alias after updating entry
{
    _id: ObjectId("51db0cf181e0e7585f000007"),
    entry_ids: [],
    name: "test"
}