Hey Jonathan,
Assume I have a Parent instance fetched from the database (Postgres)
and it has some existing Child instances (as the Parent and Child
instances were all created at the same time). The first thing I did is
call
existing_parent.update(new_parent_values)
...which updates values in the Parent instance but nothing related to
the children.
Then to update the children I did this
existing_parent.children.each do |child|
updated = child.update( { #values })
end
...which makes some changes to the existing children that are
persisted just fine.
Then I want to add a NEW child to the children collection, which I did
this way
existing_parent.children << create_child_from(data)
#create_child_from(data) creates a new Child instance in memory and
populates it with data
existing_parent.save
It is on this third operation where I get the sequence error.
Executing the reset sequence statement beforehand does indeed help, so
that solves the problem technically.
I would still love to know if there is a more efficient or elegant way
to do the same sort of thing--in fact, for all the operations of
updating the parent, then updating the existing children, then adding
a brand new child. The documentation I've been reading seems a bit
dated.
Thanks.
On Jan 14, 8:39 am, Jonathan Stott <[email protected]> wrote:
> Hi Neil
>
> Errors like that tend to occur if you've imported a whole bunch of
> pre-existing records into the database/table without updating the
> sequence. Ideally you should never need to run it, but if you have
> to, the time to do it is just after you've performed the DB import
> task. If you haven't just done that, can you explain what you did
> before the errors occured?
>
> I'm not sure DataMapper has any explicit support for this.
>
> Regards
> Jonathan
>
> On 14 January 2012 00:54, Neil Chaudhuri <[email protected]> wrote:
>
>
>
>
>
>
>
> > Apparently, the reason why updates won't save is a sequence issue as
> > found
> > here:http://railspikes.com/2009/3/6/duplicate-key-violates-unique-constraint
>
> > So I suppose I need to do something like this: select
> > setval('children_id_seq', (select max(id) + 1 from children));
>
> > I have two questions as I am not a DBA:
>
> > 1) When exactly do I do this? Every time I want to insert a child
> > record?
> > 2) Why doesn't DataMapper handle this? Maybe I am spoiled by using
> > Hibernate in the Java world, but they handle sequences fairly easily
> > by using what they call dialects. I assumed the database-specific
> > DataMapper gems would do something similar. Or maybe they do and I am
> > just missing it?
>
> > Thanks.
>
> > On Jan 13, 1:14 pm, Neil Chaudhuri <[email protected]> wrote:
> >> I have been able to create a 1:M relationship using DataMapper and
> >> update the child items as well after creation. However, I have been
> >> unable to add to that collection.
>
> >> Let's say I have
>
> >> class Parent
> >> include DataMapper::Resource
>
> >> property :id, Serial
> >> ...
>
> >> has n, :children
> >> end
>
> >> class Child
> >> include DataMapper::Resource
>
> >> property :id, Serial
> >> ...
> >> belongs_to :parent
> >> end
>
> >> Whenever I try to add to the child collection belonging to a certain
> >> parent instance, I get either DataMapper::UpdateConflictError or
> >> "DataObjects::IntegrityError: ERROR: duplicate key value violates
> >> unique constraint "children_pkey" DETAIL: Key (id)=(1) already
> >> exists.
>
> >> I have tried a whole host of approaches. The most promising seemed
> >> parent.children.create(attributes), but that led to the
> >> IntegrityError. What is the preferred approach for adding a new item
> >> to the collection?
>
> >> Thanks.
>
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