Get a list of all unique tags in mongodb

I am beginning with mongodb and have a collection with documents that look like the following

{
    "type": 1,
    "tags": ["tag1", "tag2", "tag3"]
}
{
    "type": 2,
    "tags": ["tag2", "tag3"]
}
{
    "type": 3,
    "tags": ["tag1", "tag3"]
}
{
    "type": 1,
    "tags": ["tag1", "tag4"]
}

With this, I want a set of all the tags for a particular type. For example, for type 1, I want the set of tag1, tag2, tag3, tag4 (any order).

All I could think of is to get the tags and add them to a set in python, but I wanted to know if there is a way to do it with mongodb's mapreduce or something else. Please advise.