Returning the keys of all documents in CouchDb

There's a bit of a learning curve when trying to use CouchDb's mapreduce. One of the harder parts is to write the reduce function, which can have two separate cases: called from the map functions, and called again from reduce functions.

When you emit data from map, the examples show you emitting the document, but you can emit any data structure you care to dream up in the key and value portion of the emit. I needed a mapreduce view that returned all the keys that were present in the all the documents. So if I had documents in the db in the form:

{"year": 2008, "birth_rate": 20.0 }
{"year": 2009, "birth_rate": 21.0 }
{"year": 2008, "death_rate": 20.0 }
{"year": 2009, "death_rate": 20.0 }

I wanted something that returned: ["year", "birth_rate", "death_rate"]

Here's one way to do it:

	/* this mapreduce pair returns all the keys of the attribute hash from all records in the system */
	/* the map function to return all the values of keys */
	function(doc) {
	keys = {}
	for(key in doc) {
	if (key != "_id" && key != "_rev") {
	keys[key] = 1
	}
	}
	emit(null, keys);
	}
	/* the reduce function to combine all the keys of the attribute hash in the values */
	function(keys, values, rereduce) {
	result = {}
	for(hash_name in values) {
	hash = values[hash_name];
	// merge all the hashes together
	for(key in hash) {
	result[key] = hash[key];
	}
	}
	return result;
	}

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