Question

How would I execute a query equivalent to “select top 10” in couch db?

For example I have a “schema” like so:

title   body   modified

and I want to select the last 10 modified documents.

As an added bonus if anyone can come up with a way to do the same only per category. So for:

title   category   body   modified

return a list of latest 10 documents in each category.

I am just wondering if such a query is possible in couchdb.

Answer 1

here is what you need to do.

Map function

function(doc)
{
  if (doc.category)
  {
    emit(['category', doc.category], doc.modified);
  }
}

then you need a list function that groups them, you might be temped to abuse a reduce and do this, but it will probably throw errors because of not reducing fast enough with large sets of data.

function(head, req)
{
  % this sort function assumes that modifed is a number
  % and it sorts in descending order
  function sortCategory(a,b) { b.value - a.value; }
  var categories = {};
  var category;
  var id;
  var row;
  while (row = getRow())
  {
    if (!categories[row.key[0]])
    {
      categories[row.key[0]] = [];
    }
    categories[row.key[0]].push(row);
  }
  for (var cat in categories)
  {
    categories[cat].sort(sortCategory);
    categories[cat] = categories[cat].slice(0,10);
  }
  send(toJSON(categories));  
}

you can get all categories top 10 now with

http://localhost:5984/database/_design/doc/_list/top_ten/by_categories

and get the docs with

http://localhost:5984/database/_design/doc/_list/top_ten/by_categories?include_docs=true

now you can query this with a multiple range POST and limit which categories

curl -X POST http://localhost:5984/database/_design/doc/_list/top_ten/by_categories -d '{"keys":[["category1"],["category2",["category3"]]}'

you could also not hard code the 10 and pass the number in through the req variable.

Here is some more View/List trickery.