orderByChild method
Generate a view of the data sorted by values of a particular child key.
Intended to be used in combination with startAt, endAt, or equalTo.
Implementation
Query orderByChild(String key) {
assert(key != null);
assert(!_parameters.containsKey('orderBy'));
return _copyWithParameters(
<String, dynamic>{'orderBy': 'child', 'orderByChildKey': key},
);
}