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From "Markus Jelsma" <mar...@buyways.nl>
Subject Re: M/R/M, again
Date Fri, 29 Jan 2010 01:12:53 GMT
Nicholas, thanks for your reply!


I completely agree with you that CouchDB users need to leave their RDBMS
knowledge aside, i tell the same to my co-workers but somehow i tend to
feel a need for keeping separate documents. Perhaps i'm biased by some
wiki pages that also tend to do the same. You'll recognize the case with
products and purchases where each purchase is a separate document.
Although the use-case is not entirely the same, you can get much
information on purchases in that scenario with simple map/reduce
functions.

But, if map/reduce/merge can simplify my life and others, why shouldn't it
be implemented? I'm quite sure many of us will be very grateful if such a
feature was to be available, don't you agree?

If it is not being implemented or even suggested as a real proposal - as
the list of proposals on the wiki tells us - i would have no other choice
than following the principle you speak about. For my use case, it would
not be such a bad choice because these kind of updates are only initiated
by that same user only, so there is no worry about the
multiple-document-transaction paradigm which does not exist in CouchDB.

Nevertheless, i, and possibly the core developers, sure would like to know
if there are more users that'd like to utilize a feature like me.

Perhaps i just need to be either patient or bend the use-case even more
towards a pure document oriented database which will not allow us to merge
documents in a view.


Cheers,

Nicholas Orr said:
> The other thing you could do is merge the two docs together.
> I've done this recently and boy did it make life simple.
>
> CouchDB requires a different approach than RDBMS.
> Applying RDBMS concepts to CouchDB will leave you wanting RDBMS
> features. Consider stepping back for a moment and forget RDBMS way of
> doing things and solve what you're trying to solve with CouchDB
> (Document) concepts.
>
> From what I can tell you want a username/email to map to a primaryid
> depending on the app in question.
>
> Would this work?
>
> {
> "_id": "a@a.com",
> "username": "adam",
> "type": "profile"
> "apps":
> [{"applicationId":"app2","primaryId":18},{"applicationId":"app1","primaryId":17}]
> }
>
> This usually has far reaching implications when converting from RDBMS to
> Document Store.
> I've made choices like, well if applicationId changes then I'm going to
> have to update somehow, there isn't a cascade or atomic op in CouchDB,
> so how about applicationId is not allowed to change only allowed to
> create new ones, that works :) (then if I want to delete I have a
> active/disabled flag)
>
> Food for thought
>
> Nick
>
> 2010/1/29 Markus Jelsma <markus@buyways.nl>
>
>> Hi Jan,
>>
>>
>> Thanks for your reply, but i'm afraid that i have provided a lousy
>> explanation
>> of the case i run in to. Let me explain with actual examples for i
>> believe Damien's examples do not fit my use case.
>>
>> I have a tiny database with two types of documents, profile and
>> profileApplication. The profile type has an ID which is the user's
>> e-mail address and a simply username field, nothing more (see below
>> for anatomy of both document types).
>>
>> {
>>   "_id": "markus@buyways.nl",
>>   "_rev": "1-5f7718ae8a627f4cf5b93b63420b7e1f",
>>   "type": "profile",
>>   "username": "markus17"
>> }
>> {
>>   "_id": "1d2d9db700029557666e5d260b2ea038",
>>   "_rev": "2-279daa538abc5cbb4b1524d29ce4ab53",
>>   "type": "profileApplication",
>>   "applicationId": "app2",
>>   "profileId": "markus@buyways.nl",
>>   "primaryId": 18
>> }
>>
>> The documents with profileApplication type are related to both an
>> application
>> (which i have omitted for now) and a profile. In RDBMS terms its
>> purpose would
>> be a common link table.
>>
>> The purpose for this relation is that a single profile can have a
>> different primaryId for different applications. My profile
>> (markus@buyways.nl) would have primaryId=18 for app2 and primaryId=17
>> for app1 etc.
>>
>> The goal would be to retrieve both my profile document _and_ the
>> primaryId that goes with my profile for app1 or app2, ideally the
>> query would be key=["markus@buyways.nl", "app1"], but this is
>> currently not possible.
>>
>> There are two things i can do now:
>> 1) retrieve the profile first and then fetch the primaryId for the
>> application
>> i need, but this takes two requests and manually merging of the
>> profile data
>> and primaryId;
>>
>>
>> http request 1:
>> http://localhost:5984/test/markus@buyways.nl
>>
>> output:
>> {"_id":"markus@buyways.nl
>> ","_rev":"1-5f7718ae8a627f4cf5b93b63420b7e1f","type":"profile","username":"markus17"}
>>
>> http request 2:
>>
>> http://localhost:5984/test/_design/profiles/_view/getPrimaryByEmailAndApplication?key=[%22markus@buyways.nl%22
>> ,
>> %20%22app1%22]
>>
>> output:
>> {"total_rows":4,"offset":2,"rows":[
>> {"id":"f51b92f4a59de0e28641375637a73050","key":["markus@buyways.nl
>> ","app1"],"value":17}
>> ]}
>>
>> It's clear that i need to merge the value of the second request with
>> the document received by the first.
>>
>>
>> 2) fetch the profile and all related primaryIds in one go, this is one
>> single
>> requests but i also get primaryId's for apps that i don't need so this
>> fetches
>> more data and also needs clientside merging after i filtered out the
>> app i need.
>>
>>
>> http request 1:
>>
>> http://localhost:5984/test/_design/profiles/_view/getProfileApplications?startkey=[%22markus@buyways.nl%22]&endkey=[%22markus@buyways.nl%22
>> ,
>> %20%22zzz%22]
>>
>> output:
>> {"total_rows":6,"offset":3,"rows":[
>> {"id":"markus@buyways.nl","key":["markus@buyways.nl",1],"value":null},
>> {"id":"f51b92f4a59de0e28641375637a73050","key":["markus@buyways.nl
>> ","app1",2],"value":17},
>> {"id":"1d2d9db700029557666e5d260b2ea038","key":["markus@buyways.nl
>> ","app2",2],"value":18}
>> ]}
>>
>> It's clear that i need to filter my profile document and the
>> profileApplication document for the app i want (app1). The bad thing
>> here is
>> that i do not get my profile document in the value (although i can
>> emit it but
>> that's), if i include_docs i'll also get a lot of extra data on the
>> documents
>> i don't need, here it's just one document but i can be many.
>>
>>
>>
>> Both techniques work and have their pros and cons. But do you agree
>> that it would be much more convenient if we could simply construct
>> views that carry merged or combined documents using
>> key=["markus@buyways.nl","app1"].
>>
>> Am i correct to assume i cannot achieve the goal stated above without
>> either
>> Chris' technique or merging of documents in one single view?
>>
>> Please forgive me if i somehow didn't understand Damien's example but
>> i believe that deals with arithmetic instead of merging complex data
>> structures.
>> I also didn't (yet?) feel that the new 0.11 linked documents feature
>> will help
>> me out here. Also, i wish to keep this data in separate documents,
>> keeping an
>> array within the profile document isn't really the best approach i
>> think.
>>
>>
>>
>> Cheers,
>>
>>
>> >See http://damienkatz.net/2008/02/incremental_map.html and
>> >http://damienkatz.net/2008/02/incremental_map_1.html and the
>> >comments on both.
>>
>> Markus Jelsma - Technisch Architect - Buyways BV
>> http://www.linkedin.com/in/markus17
>> 050-8536620 / 06-50258350




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