couchdb-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Patrick Barnes <>
Subject Re: Conditional joins
Date Fri, 17 Sep 2010 13:57:02 GMT
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 

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.


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 where parent.status=1;
> I've read the 'join' documentation (e.g.
> 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":"","status":1},
> {"_id":"45a6951b34a6f45bd44fd169c00046db","_rev":"1-37eaf0c4944dac9f5fe07b25c0b68b1d","type":"domain","domain":"","status":0},
> {"_id":"45a6951b34a6f45bd44fd169c00053d9","_rev":"3-729f46c86ee4984941939c65787126fb","type":"user","domain":"","user":"foo","password":"test123"},
> {"_id":"45a6951b34a6f45bd44fd169c00060c8","_rev":"1-46997eba12e7adbb6e9266a1fe83c42d","type":"user","domain":"","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 but
> not for
> Any help would be appreciated.
> Thanks!
> Simon
> --
> ***** Email confidentiality notice *****
> This message is private and confidential. If you have received this message in error,
please notify us and remove it from your system.
> Simwood eSMS Limited is a limited company registered in England and Wales. Registered
number: 03379831. Registered office: c/o HW Chartered Accountants, Keepers Lane, The Wergs,
Wolverhampton, WV6 8UA. Trading address: Falcon Drive, Cardiff Bay, Cardiff, CF10 4RU.

View raw message