> db.a.runCommand("listIndexes")
{
	"cursor" : {
		"id" : NumberLong(0),
		"ns" : "test.$cmd.listIndexes.a",
		"firstBatch" : [
			{
				"v" : 1,
				"key" : {
					"_id" : 1
				},
				"name" : "_id_",
				"ns" : "test.a"
			},
			{
				"v" : 1,
				"key" : {
					"a" : 1
				},
				"name" : "a_1",
				"ns" : "test.a"
			},
			{
				"v" : 1,
				"key" : {
					"b" : 1
				},
				"name" : "b_1",
				"ns" : "test.a"
			}
		]
	},
	"ok" : 1
}
> db.a.runCommand("listIndexes", {filter:{"key.a":{$exists:1}}})
{
	"cursor" : {
		"id" : NumberLong(0),
		"ns" : "test.$cmd.listIndexes.a",
		"firstBatch" : [
			{
				"v" : 1,
				"key" : {
					"a" : 1
				},
				"name" : "a_1",
				"ns" : "test.a"
			}
		]
	},
	"ok" : 1
}