ActiveRecord: moving all children to a new parent

rubyonrails-talk
1

I have a models Parent and Child, an the following association:

Parent   has_many :children, dependent: :destroy

Child   belongs_to :parent

Further, I have two Parent instances:

  pfrom = Parent.find_by_id(from_id)   pto = Parent.find_by_id(to_id)

My goal is to transfer all children from pto to pfrom, and then delete pto.

The first part seems to be easy:

  pfrom.children.each { |ch| ch.update_attributes!(parent_id: pto.id }

If I run *only* this code, I can see that pto indeed contains now the children formerly belonging to pfrom, and iterating over pfrom shows that there are no children.

HOWEVER, if I add the following line:

  pfrom.destroy

I can see (from the SQL statements which are issued by this call), that all the former pfrom children are deleted!

It somehow seems as if this information has been "cached". Could this be the case? How then would I correctly implement the "move".

2

I have a models Parent and Child, an the following association:

Parent has_many :children, dependent: :destroy

Child belongs_to :parent

Further, I have two Parent instances:

pfrom = Parent.find_by_id(from_id) pto = Parent.find_by_id(to_id)

My goal is to transfer all children from pto to pfrom, and then delete pto.

The first part seems to be easy:

pfrom.children.each { |ch| ch.update_attributes!(parent_id: pto.id }

If I run *only* this code, I can see that pto indeed contains now the children formerly belonging to pfrom, and iterating over pfrom shows that there are no children.

HOWEVER, if I add the following line:

pfrom.destroy

I can see (from the SQL statements which are issued by this call), that all the former pfrom children are deleted!

It somehow seems as if this information has been "cached". Could this be the case? How then would I correctly implement the "move".

Reload the parent that you wish to destroy before you destroy it. Also, maybe it would be enough to set the "dead" parent's children array to . While the key is stored only on one side, when the parent record is initialized, its children are instantiated in memory. You are correct, they are cached.

Walter

3

Walter Davis wrote in post #1152668:

pfrom = Parent.find_by_id(from_id) children formerly belonging to pfrom, and iterating over pfrom shows the case? How then would I correctly implement the "move".

Reload the parent that you wish to destroy before you destroy it.

I was not aware of the reload method! Thank you for pointing this out. So this would be

  pfrom.reload.destroy

Also, maybe it would be enough to set the "dead" parent's children array to .

Interesting idea. I think, 'reload' is nicer, because it is more likely that this part of the interface won't change when a new version of Rails is coming. Changing the children-array looks a bit like a hack to me (we need to know that they are stored in an array). But still I'm curious: How do I explicitly manipulate the childrens array? I didn't find a suitable method in the Active Record docs, and I don't expect that something like

  pfrom.children=

would do it.

Ronald

4

That’s actually exactly what will do it. :slight_smile:

Running that will do a single UPDATE query to set all the involved parent_id columns to NULL. So one way to implement the swap is:

saved_children = pfrom.children.to_a

pfrom.children =

pto.children = saved_children

NOTE: this won’t work if there are children on pto already. For that, try pto.children = pto.children + saved_children.

–Matt Jones

5

I am stunned!!!!!

Every day I like Rails more....

Thanks a lot!

Ronald

6

Matt Jones wrote in post #1152815:

saved_children = pfrom.children.to_a pfrom.children = pto.children = pto.children + saved_children.

I finally found the time to rewrite this part of my application according to this suggestion, but I now get an error "can't modify frozen Hash", when I try to add the saved children. The actual code which I am using is here:

# tempdict is "pfrom" and targetdict is "pto"        targetdict=Dict.find_by_id(....)       # creating and saving a tempdict together with several children, i.e. cards       tempdict=Dict.new(....)       tempdict.save!       # Code for creating and adding the children omitted for brevity       ....       cards_to_add=tempdict.cards.to_a       tempdict.cards=       targetdict.cards += cards_to_add

I find it strange that I get the error on the last line. The error message is usually an indication that I am trying to save something which has been deleted already, but in this case, no deletion had been done on 'targetdict' before.

Any idea, where this error could come from?

7

pfrom.children=

would do it.

That’s actually exactly what will do it. :slight_smile:

Running that will do a single UPDATE query to set all the involved parent_id columns to NULL. So one way to implement the swap is:

saved_children = pfrom.children.to_a

pfrom.children =

pto.children = saved_children

This will depend on what you have set the :dependant option on the association to - it would do this if you have set the option to nullify (the default), but it would destroy the children if it was :destroy or :delete_all

Fred

8

Ah, I have set it to :destroy!

But why do I get the error when I assign to pto.children? I would expect such an error then when pfrom is deleted.

Ronald

9

You tell me; you’re the one who can see the stack trace. :slight_smile:

Maybe something that it’s modifying has been destroyed?

–Matt Jones