query.json

{
    "hometown": {
        "$select": {
            "query": {
                "className": "Team",
                "where": {
                    "winPct": {
                        "$gt": 0.5
                    }
                }
            },
            "key": "city"
        }
    }
}

query.rb

# experimental

# current api
users = Parse::Query.new("_User").tap do |user_query|
  user_query.eq("hometown", {
    "$select" => {
      "query" => Parse::Query.new("Team").tap do |team_query|
        team_query.greater_than("winPct", 0.5)
      end,
      "key" => "city"
    }  
  })
end.get

# proposed api
users = Parse::Query.new("_User").tap do |user_query|
  user_query.select_query("hometown", "Team").tap do |team_query|
    team_query.greater_than("winPct", 0.5)
    team_query.key = "city"
  end
end.get

query.sh

# If you have a class containing sports teams and you store a user's hometown in the user class, 
# you can issue one query to find the list of users whose hometown teams have winning records. The query would look like:

curl -X GET \
  -H "X-Parse-Application-Id: FOO" \
  -H "X-Parse-REST-API-Key: BAR" \
  -G \
  --data-urlencode 'where={"hometown":{"$select":{"query":{"className":"Team","where":{"winPct":{"$gt":0.5}}},"key":"city"}}}' \
  https://api.parse.com/1/classes/_User