MongoDB - finding entries using a nested dictionary

I have a MongoDB that I use with python. My entries that looks as follows:

{
   a: something
   b: something
   c: {
        d: something
        e: something
        f: something
       }
}

I want to query for entries that have a specific values for d and e but I don't care about the value in f.

I tried not specifying f at all (similarly to when I don't care about the b value where I just not adding it to the query):

{
   a: a_value
   b: b_value
   c: {
        d: d_value
        e: e_value
       }
}

I also tried to use:

{
   a: something
   b: something
   c: {
        d: something
        e: something
        f: { $exists : 1 }
       }
}

but none of these worked (in fact, I got no results at all)

How my query shall look like?

thanks!