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From Andy Dorman <ador...@ironicdesign.com>
Subject Re: How to implement multiple-tables/collections using CouchDB?
Date Thu, 11 Sep 2014 20:07:42 GMT
On 09/11/2014 02:17 PM, Matthieu Rakotojaona wrote:
> Excerpts from Eric B's message of 2014-09-11 16:19:21 +0200:
>> I've been reading up on CouchDB and am very confused as to its application
>> in a real world web application.  I can see it's benefit on a subset of
>> data, but as a primary DB for a web application, I can't say that I can see
>> how to do things.
>>
>> Coming from my RDBMS background, I would build a basic shopping cart web
>> app with a bunch of tables:
>>   - users
>>   - permissions
>>   - items
>>   - inventory
>>   - clients
>>   - orders
>>   - etc...
>>
>> I realize that NoSQL allows me to denormalize a lot of that data (which is
>> fine) in order to speed up processing/etc, and the concept of "tables" no
>> longer applies.    I'm fine with that.  But where I get lost is how to
>> organize similar datasets together in CouchDB.  In MongoDB, they have
>> Collections - akin to tables - to help separate data.
>>
>> I don't see anything equivalent in CouchDB.  Which would mean to throw all
>> my data into a single "bag"/collection and rely entirely on map
>> functions/views to help organize data.  For instance, to retrieve all users
>> from the system, I would need a map function that does something like:
>>   emit all docs that have a username field.
>>
>> But then what happens if at a later point in time, I create another
>> document that has a username field (which isn't a user)?  It will break my
>> code.
>>
>> So then my second option becomes to assign a "document-type" key to every
>> document and then filter upon that.  Where my "document-type" key is akin
>> to an organizational/collection name.  It's definitely better, but still
>> seems a little odd.
>>
>> The whole process seems very disorganized.
>>
>> I understand the concept that NoSQL is exactly that - a key/value store,
>> where structure is omitted. But I would have expected at the very least
>> some organization ability - no?
>>
>> Am I missing something basic/obvious in CouchDB?  Or is the concept to use
>> separate DBs everytime you want to organize similar data together?  That
>> also seems a little odd too.
>>
>> Thanks,
>>
>> Eric
>
> In CouchDB, the organization and planning you have to do is not in
> how you want your data stored, it's in how you're going to
> retrieve/query it. If you need to display the last 5 ordered items of a
> user a lot (as in, "here are your last 4 purchases"), you're going to
> write a view that allows you to do that easily. If you need to list all
> the orders that happened since beginning of the month, it will be the
> same. This is where the shift can be, coming from RDBMS.
>
> To be more precise, the map process happens somewhat like that (in pseudocode):
>
>      for doc in new_docs():
>        emit some_property from doc
>
> The key here is that CouchDB is targeted towards querying sorted things.
> You have to make it so that property sorts.
>
> For example, for the second example, you should emit the order date in a
> sortable date (use ISO8601). For the first example, on each order, emit
> something like "user_id:order_date" with a sortable order_date. When
> user_id 4 connects and fetches its 5 last ordered items, CouchDB will
> just iterate on the 5 relevant entries that you have indexed.
>
> "Indexed" here is the important word: "new_docs" evaluate to all docs
> when querying the view for the first time, but then it only evaluates to
> "docs that have changed/been added since the last time the view was
> queried". Results are incrementally stored for efficient querying.
>

I realize it is Cloudant, but would work as well for CouchDB...

I have been working with RDBMS since the late 70's and making the shift 
to a document db like CouchDB has been a bit of a challenge...but at 
least one light came on for me when I saw this webinar last week.

https://cloudant.com/handling-relational-data-with-cloudant-webinar-playback/

-- 
Andy Dorman
Ironic Design, Inc.
AnteSpam.com, ComeHome.net

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