The basic tenets of views are:
1. Map/reduce functions must be idempotent, and look only at the
document/data passed to it.
2. Emitted keys get sorted onto a single axis, and with view queries you
can retrieve a single given key, multiple given keys, or a contiguous
subset of keys with a given start and end.
For other scenarios where a link between documents needs to be displayed
and a subset of documents retrieved (ie for a one->many relationship
like posts->comments), both the independent and dependent documents have
to emit a key that can place them together appropriately in the view
axis. (so a post could emit [doc._id, 0] and a comment could emit
[doc.post_id, 1])
The fundamental issue with your scenario is tenet #1 - there's normally
no way for a 'child' to know what the status of the 'parent' is. This
also causes problems for tenet #2, because if you want to emit for
instance the status in the key, #1 means that the 'child' can't emit
that same key to be adjacent to the parent. (it won't know what to emit)
Basically, I can think of two ways to solve this:
A. Have the necessary fields of the 'parent' be stored also in the
dependent 'child' - i.e. doc.parent_status. Then you can make a view
that either only shows active docs, or is keyed like eg [status,
parent_id, 0|1]
This approach works if you have large numbers of docs that might be need
to be returned at a time, but the docs don't change that attribute very
often. (because any status update to the parent needs to be cascaded to
children)
B. Use two requests like Wout suggested - In the first request fetch the
parents. (out of a view that allows you to select by status as you need)
In the second request, fetch the children of those parents. (on a second
view, using multiple keys).
This does not need extra data in the view, but if there are too many
keys requested in the second view it can be inefficient.
B is less hassle, unless you have large numbers of docs read at once,
and don't change the docs very often.
HTH,
-Patrick
On 17/09/2010 11:04 PM, Simon Woodhead wrote:
Hi folks,
I'm pretty green with views and have a need to do a join with results
from one side returned if a condition is met on the other. In SQL I'd
do:
select * from child join parent on child.id=parent.id where parent.status=1;
I've read the 'join' documentation (e.g.
http://www.cmlenz.net/archives/2007/10/couchdb-joins) and can return
documents of multiple types with matching keys ok. What I can't do is
do the same but only if the parent matches a condition.
Specifically, my data is like:
{"45a6951b34a6f45bd44fd169c0003f9c","_rev":"1-6c88818b43d562c45e452bf00db06ee2","type":"domain","domain":"test.com","status":1},
{"_id":"45a6951b34a6f45bd44fd169c00046db","_rev":"1-37eaf0c4944dac9f5fe07b25c0b68b1d","type":"domain","domain":"test2.com","status":0},
{"_id":"45a6951b34a6f45bd44fd169c00053d9","_rev":"3-729f46c86ee4984941939c65787126fb","type":"user","domain":"test.com","user":"foo","password":"test123"},
{"_id":"45a6951b34a6f45bd44fd169c00060c8","_rev":"1-46997eba12e7adbb6e9266a1fe83c42d","type":"user","domain":"test2.com","user":"bar","password":"test123"}
I want to be be able to query a view by domain and see all users for
it if the status is 1. So in this case I'd see users for test.com but
not for test2.com.
Any help would be appreciated.
Thanks!
Simon
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