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?
In order to retrieve data with CouchDB, we use a process called MapReduce, to create views. A view contains rows of data that is sorted by the row's key (you might use date as a key, for example, to sort your data based on the date). MapReduce is a combination of two concepts Map and Reduce.
Views are useful for many purposes: Filtering the documents in your database to find those relevant to a particular process. Extracting data from your documents and presenting it in a specific order.
There are two employees in our "employees" database. Now, Open Fauxton and go to all documents where you see a block named New View. View is created now.
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.
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 [email protected]"
, "type": "customer"
, "name": "Jason"
}
Example order:
{ "_id": "abcdef123456"
, "type": "order"
, "for_customer": "customer [email protected]"
}
I have conveniently used the customer ID as the document _id
but the important thing is that both docs know the customer's identity.
The goal is a map query, where if you specify ?key="customer [email protected]"
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 [email protected]", 1], ...customer doc...
["customer [email protected]", 2], ...customer's order...
["customer [email protected]", 2], ...customer's other order.
... etc...
["customer [email protected]", 1], ... different customer...
["customer [email protected]", 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.
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