code: function(key, query) {
          var queryJSON = query.toJSON();
          queryJSON.className = query.className;
          this._addCondition(key, '$inQuery', queryJSON);
          return this;
        }
docs: Add a constraint that requires that a key's value matches a AV.Query
  constraint.
  @param {String} key The key that the contains the object to match the
  query.
  @param {AV.Query} query The query that should match.
  @return {AV.Query} Returns the query, so you can chain this call.