Question

In MongoDB, using $type, it is possible to filter a search based on if the field matches a BSON data type (see DOCS).

For eg.

db.posts.find({date2: {$type: 9}}, {date2: 1})

which returns:

{ 
    "_id" : ObjectId("4c0ec11e8fd2e65c0b010000"), 
    "date2" : "Fri Jul 09 2010 08:25:26 GMT" 
}

I need a query that will tell me what the actual type of the field is, for every field in a collection. Is this possible with MongoDB?

Answer 1

OK, here are some related questions that may help:

Get all field names in a collection using map-reduce.

Here’s a recursive version that lists all possible fields.

Hopefully that can get you started. However, I suspect that you’re going to run into some issues with this request. There are two problems here:

1. I can’t find a “gettype” function for JSON. You can query by $type, but it doesn’t look like you can actually run a gettype function on a field and have that maps back to the BSON type.
2. A field can contain data of multiple types, so you’ll need a plan to handle this. Even if it’s not apparent Mongo could store some numbers as ints and others floats without you really knowing. In fact, with the PHP driver, this is quite possible.

So if you assume that you can solve problem #1, then you should be able to solve problem #2 using a slight variation on “Get all field Names”.

It would probably look something like this:

"map" : function() { for (var key in this) { emit(key, [ typeof value[key] ]); } }
"reduce" : function(key, stuff) { return (key, add_to_set(stuff) ); }

So basically you would emit the key and the type of key value (as an array) in the map function. Then from the reduce function you would add unique entries for each type.

At the end of the run you would have data like this

{"_id":[255], "name" : [1,5,8], ... }

Of course, this is all a lot of work, depending on your actual problem, you may just want to ensure (from your code) that you’re always putting in the right type of data. Finding the type of data after the data is in the DB is definitely a pain.