Selecting specific fields from a relationship

I'm building a webapp on node.js (sails) with waterline (using mysql adapter).

I've managed to get relationships working like this:

//Post.js attributes
    post_statistics: {
        collection: 'postStatistics',
        via: 'post'
    }

I can easily find the relationship records with the line:

var query = {user:userId}; // Just filtering by the user
query.is_public = Post.POSTS_ONLY_ACTIVE; // Only showing active posts, boolean
query.select = ['id','title','content']; // id is only needed for me to get the join working
Post.find(query).populate('post_statistics').exec(cb);

Everything works fine but I need to only select specific fields from post_statistics. This does not seem to do anything, though, I'd expect it to work:

Post.find(query).populate('post_statistics',{select:['post','user','id']}).exec(cb);

Adding the field names to the initial select is even worse since the fields already target the post table.

Any sane way to make this work or am I going to have to write the whole query?