Need Explanation of couchdb reduce function
- by Alan
From http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views
The couchdb reduce function is defined as
function (key, values, rereduce) {
return sum(values);
}
key will be an array whose elements
are arrays of the form [key,id]
values will be an array of the values
emitted for the respective elements
in keys
i.e. reduce([ [key1,id1], [key2,id2], [key3,id3] ], [value1,value2,value3], false)
I am having trouble understanding when/why the array of keys would contain different key values. If the array of keys does contain different key values, how would I deal with it?
As an example, assume that my database contains movements between accounts of the form.
{"amount":100, "CreditAccount":"account_number", "DebitAccount":"account_number"}
I want a view that gives the balance of an account.
My map function does:
emit( doc.CreditAccount, doc.amount )
emit( doc.DebitAccount, -doc.amount )
My reduce function does:
return sum(values);
I seem to get the expected results, however I can't reconcile this with the possibility that my reduce function gets different key values.
Is my reduce function supposed to group key values first? What kind of result would I return in that case?
