ActiveRecord delete_all method updating instead of deleting

Question

I'm using Rails polymorphic associations, that way some models have many cash_histories children, like this:

has_many :cash_histories, as: :cashable 

But when I try to delete all cash histories from a parent @resource, like this:

@resource.cash_histories.delete_all 

I get the following query:

UPDATE "cash_histories" SET "cashable_id" = NULL WHERE "cash_histories"."cashable_id" = $1 AND "cash_histories"."cashable_type" = $2  [["cashable_id", 1], ["cashable_type", "ServiceOrder"]] 

I can't understand this behavior, setting relationship id to null instead of removing, that will result in dead rows in my table. Why is that happening?

I'm using Rails 4.1.

Answer 1

From the Rails API docs for delete_all:

Deletes all the records from the collection. For has_many associations, the deletion is done according to the strategy specified by the :dependent option. Returns an array with the deleted records.

If no :dependent option is given, then it will follow the default strategy. The default strategy is :nullify. This sets the foreign keys to NULL. For, has_many :through, the default strategy is delete_all.

So you you just need to set the :dependent option on your has_many to either :delete_all or :destroy, depending on what behavior you want.

has_many :cash_histories, as: :cashable, dependent: :delete_all 

From the Rails API docs for has_many:

Objects will be in addition destroyed if they're associated with dependent: :destroy, and deleted if they're associated with dependent: :delete_all.