Hi everyone else (and Jeff),
Jeff dropped by #datamapper, and we got him sorted out.
This particular problem can be addressed by a query like this:
Issuance.all("url.host"=>"http://www.datamapper.org";)
I just added an entry to the FAQ on this subject as well:
http://github.com/datamapper/dm-core/wikis/preview#query_relationships
If anyone has questions please feel free to drop by #datamapper! Also
the FAQ is open for people who have questions or answers!
-knowtheory
On May 1, 12:07 am, Jeff Pollard <[email protected]> wrote:
> I have a pretty simple join that I'm trying to do in a DB-friendly way.
> Here are my models:
>
> class Issuance
> include DataMapper::Resource
>
> property :id, Serial
> property :issue_id, Integer
> property :url_id, Integer
> belongs_to :issue, :child_key => [:issue_id]
> belongs_to :url, :child_key => [:url_id]
> end
>
> class Url
> include DataMapper::Resource
>
> property :id, Serial
> property :host, String
>
> [...]
>
> has n, :issuances
> has n, :issues, :through => :issuances
> end
>
> I want a list of all the Issuance models whose Urls belong to a certain
> domain. I've tried the following statements:
>
> Issuance.all(:url => {:host => "www.datamapper.org"})
>
> Url.all(:host => "www.datamapper.org").issuances
>
> In each of these cases, datamapper is doing it MOSTLY correct, by issuing
> these queries:
>
> SELECT `id` FROM `urls` WHERE `host` = 'www.datamapper.org'
>
> SELECT `id`, `issue_id`, `url_id`, `host`, `found` FROM `issuances` WHERE
>
> > `url_id` IN (1, 2, 3, 4, etc...)
>
> I have lots of URLs (thousands) in the DB, so the "AND `id` IN (1, 2, 3...)"
> part lists all the url_ids who are onwww.datamapper.orghost.
>
> I appreciate DM using the IN() operator to avoid the N+1 problem, but in
> this case what I really want it to do is issue only 1 SQL query and JOIN, to
> the effect of:
>
> SELECT * FROM issuances LEFT JOIN urls ON urls.id = issuances.url_id WHERE
>
> > urls.host = 'www.datamapper.org'LIMIT 10;
>
> Is that possible or is this a feature for future releases?
>
> -J
>
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