CODEWITHSUNDEEP
pythonjsoncouchdb
ohblahitsme
ohblahitsme

Reputation: 1061

Add a field to existing document in CouchDB

I have a database with a bunch of regular documents that look something like this (example from wiki):

{
 "_id":"some_doc_id",
 "_rev":"D1C946B7",
 "Subject":"I like Plankton",
 "Author":"Rusty",
 "PostedDate":"2006-08-15T17:30:12-04:00",
 "Tags":["plankton", "baseball", "decisions"],
 "Body":"I decided today that I don't like baseball. I like plankton."
}

I'm working in Python with couchdb-python and I want to know if it's possible to add a field to each document. For example, if I wanted to have a "Location" field or something like that.

Thanks!

Upvotes: 3

Views: 6506

Answers (1)

Hans
Hans

Reputation: 2910

Regarding IDs

Every document in couchdb has an id, whether you set it or not. Once the document is stored you can access it through the doc._id field.

If you want to set your own ids you'll have to assign the id value to doc._id. If you don't set it, then couchdb will assign a uuid.

If you want to update a document, then you need to make sure you have the same id and a valid revision. If say you are working from a blog post and the user adds the Location, then the url of the post may be a good id to use. You'd be able to instantly access the document in this case.

So what's a revision

In your code snippet above you have the doc._rev element. This is the identifier of the revision. If you save a document with an id that already exists, couchdb requires you to prove that the document is still the valid doc and that you are not trying to overwrite someone else's document.

So how do I update a document

If you have the id of your document, you can just access each document by using the db.get(id) function. You can then update the document like this: 

doc = db.get(id)
doc['Location'] = "On a couch"
db.save(doc)

I have an example where I store weather forecast data. I update the forecasts approximately every 2 hours. A separate process is looking for data that I get from a different provider looking at characteristics of tweets on the day.

This looks something like this. 

doc = db.get(id)
doc_with_loc = GetLocationInformationFromOtherProvider(doc) # takes about 40 seconds. 
doc_with_loc["_rev"]  = doc["_rev"]
db.save(doc_with_loc) # This will fail if weather update has also updated the file.

If you have concurring processes, then the _rev will become invalid, so you have to have a failsave, eg. this could do: 

doc = db.get(id)
doc_with_loc = GetLocationInformationFromAltProvider(doc)
update_outstanding = true
while update_outstanding:
    doc = db.get(id) //reretrieve this to get 
    doc_with_loc["_rev"]  = doc["_rev"]
    update_outstanding = !db.save(doc_with_loc)

So how do I get the Ids?

One option suggested above is that you actively set the id, so you can retrieve it. Ie. if a user sets a given location that is attached to a URL, use the URL. But you may not know which document you want to update - or even have a process that finds all the document that don't have a location and assign one.

You'll most likely be using a view for this. Views have a mapper and a reducer. You'll use the first one, forget about the last one. A view with a mapper does the following:

It returns a simplyfied/transformed way of looking at your data. You can return multiple values per data or skip some. It gives the data you emit a key, and if you use the _include_docs function it will give you the document (with _id and rev alongside).

The simplest view is the default view db.view('_all_docs') this will return all documents and you may not want to update all of them. Views for example will be stored as a document as well when you define these.

The next simple way is to have view that only returns items that are of the type of the document. I tend to have a _type="article in my database. Think of this as marking that a document belongs to a certain table if you had stored them in a relational database.

Finally you can filter elements that have a location so you'd have a view where you can iterate over all those docs that still need a location and identify this in a separate process. The best documentation on writing view can be found here.

Upvotes: 6

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