database - Using map reduce in CouchDB to output fewer rows

Question

Lets say you have two document types, customers and orders. A customer document contains basic information like name, address etc. and orders contain all the order information each time a customer orders something. When storing the documents, the type = order or the type = customer.

If I do a map function over a set of 10 customers and 30 orders it will output 40 rows. Some rows will be customers, some will be orders.

The question is, how do I write the reduce, so that the order information is "stuffed" inside of the rows that has the customer information? So it will return 10 rows (10 customers), but all the relevant orders for each customer.

Basically I don't want separate records on the output, I want to combine them (orders into one customer row) and I think reduce is the way?

Answer 1

This is called view collation and it is a very useful CouchDB technique.

Fortunately, you don't even need a reduce step. Just use map to get the customers and their orders "clumped" together.

Setup

The key is that you need a unique id for each customer, and it has to be known both from customer docs and from order docs.

Example customer:

{ "_id": "customer me@example.com"
, "type": "customer"
, "name": "Jason"
}

Example order:

{ "_id": "abcdef123456"
, "type": "order"
, "for_customer": "customer me@example.com"
}

I have conveniently used the customer ID as the document _id but the important thing is that both docs know the customer's identity.

Payoff

The goal is a map query, where if you specify ?key="customer me@example.com" then you will get back (1) first, the customer info, and (2) any and all orders placed.

This map function would do that:

function(doc) {
  var CUSTOMER_VAL = 1;
  var ORDER_VAL    = 2;
  var key;

  if(doc.type === "customer") {
    key = [doc._id, CUSTOMER_VAL];
    emit(key, doc);
  }

  if(doc.type === "order") {
    key = [doc.for_customer, ORDER_VAL];
    emit(key, doc);
  }
}

All rows will sort primarily on the customer the document is about, and the "tiebreaker" sort is either the integer 1 or 2. That makes customer docs always sort above their corresponding order docs.

["customer me@example.com", 1], ...customer doc...
["customer me@example.com", 2], ...customer's order...
["customer me@example.com", 2], ...customer's other order.
... etc...
["customer another@customer.com", 1], ... different customer...
["customer another@customer.com", 2], ... different customer's order

P.S. If you follow all that: instead of 1 and 2 a better value might be null for the customer, then the order timestamp for the order. They will sort identically as before except now you have a chronological list of orders.