# Updating children in a has\_many relationship

**URL:** <https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139>\
**Category:** Rails\
**Created:** [October 30, 2008, 2:24pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139 "2008-10-30T14:24:39Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 30, 2008, 2:24pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/1 "2008-10-30T14:24:39Z")

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If a parent has\_many children, The children can be set using  
parent.children\_ids = [] which is very cool. If the parent is a new  
record, nothing will happen until the parent is saved. If the parent  
is an existing record, the children will be immediately updated  
regardless of whether the parent has been modified or validated. In  
fact, the parent model will have no indication that the children have  
been modified at all.

Am I the only one that thinks this is odd behavior? It seems like  
there should be a more consistent approach.

Thanks,  
Tom

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 30, 2008, 4:33pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/2 "2008-10-30T16:33:37Z")

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I don’t think it’s odd at all… the only reason it’s not saved until  
the Parent is initially saved is if the parent has no ID, how cna you  
assign parent\_id to the child ( or the join table ).

If you’re modifying the children w/o touching the parent… say  
categories and posts in those categories… It would be up to the  
category model to validate any changes you make to it… and if you  
changed a post in one of those categories… or changed the category  
the post belonged to… why shouldn’t it be changed right away? Why  
would I need to re-validate the category?

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 30, 2008, 6:06pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/3 "2008-10-30T18:06:22Z")

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Why shouldn’t it be changed right away? Because the parent may be  
invalid.

Really, the problem is consistency. If are doing an update to the  
parent and children and the parent is invalid, the children may or may  
not be affected. The consistent way to handle it would be to not  
update the children until the parent is saved. IMHO

Thanks,  
Tom

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 31, 2008, 8:01pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/4 "2008-10-31T20:01:11Z")

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I disagree… that should be up to the developer to check… like my  
example, why do I need to validate the category, each time I post?

My feeling is that if it’s something that you need to have, then  
create a before\_filter and have it validate, otherwise you’re  
validating things that never change…

although I can see your argument for consistency… I still have no  
problem with the way it is now…

That being said, my opinion rarely matter 😛

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 31, 2008, 8:40pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/5 "2008-10-31T20:40:24Z")

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Same thing though… saving a category doesn’t necessarily mean I  
need to save every post… so you have to choose the option that is  
less stressful… imagine a category with 10,000 posts… updating  
the category would require you to save 10,000 posts…

same with any parent-child relationship…

I haven’t played with it in awhile, so I can comment specifically on  
your example with calling category.post\_ids = post.id auto saving it,  
unless you’re calling it during a create call… to me that would  
seem incorrect…

That being said, post.category\_ids is the way it would need to be  
done… because the posts are assigned the category… in this case  
calling that SHOULD NOT save the post, and should require me to save  
it manutally…

if I do post.create( params[:post] ) and one of the params is  
category\_ids[1,4,6] then on the create the post is saved… if I’m  
in the middle of am action and go post.category\_ids = 1,2,3 then it  
SHOULD NOT save until I call post.save

I think I just confused myself.

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 31, 2008, 9:40pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/6 "2008-10-31T21:40:46Z")

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That isn’t exactly how it works. If you have a category with 10,000  
posts, you would use the method “category.posts \<\< post”. This has  
the same inconsistency that if the category is an existing category,  
the post is immediately saved. If the category is a new category, the  
post will not be saved until either you call “category.save” or  
“post.save”.

In a parent-child relationship, the parent will automatically save the  
child if the child is added to the child collection. If the parent is  
unsaved, it stays associated with the parent until the parent is  
saved. Since you can’t always save the child, it would seem to be  
more consistent to always wait to save the child until either the  
parent or the child is explicitly saved. My two cents!

Thanks,  
Tom

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 31, 2008, 8:25pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/7 "2008-10-31T20:25:47Z")

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Even in the case you mention, for adding a category to a post, I would  
think you would use “past.category = category”, or the  
“post.category\_id = category.id” method – neither method would cause  
the post to save until you called “post.save”.

However, if you were to call “category.post\_ids = post.id”, it would  
immediately save the post – now “post.save” is not required. Oh  
wait! If the category was a NEW category it wouldn’t save the post  
and now you would need to call “post.save”.

The inconsistent behavior makes it hard for me to use this very cool  
method. Not a lot of people chiming in so sw0rdfish, I’m thinking my  
opinion matters even less!

Thanks,  
Tom

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 3, 2008, 8:10pm UTC](https://www.ruby-forum.com/t/updating-children-in-a-has-many-relationship/152139/8 "2008-11-03T20:10:51Z")

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yeah I see what you’re saying now…

I’ve run into it a few times where changes aren’t saved, and it turns  
out to be as a result of what you mention…

Fair enough argument… you should bring it up to the big boys up  
top. DHH, Rick or James or someone…

It does make sense.