code: function(key, value) {
        if (_.isUndefined(value)) {
          return this.doesNotExist(key);
        } 
  
        this._where[key] = Parse._encode(value);
        return this;
      }
docs: Add a constraint to the query that requires a particular key's value to
  be equal to the provided value.
  @param {String} key The key to check.
  @param value The value that the Parse.Object must contain.
  @return {Parse.Query} Returns the query, so you can chain this call.