Result Sets
Description — How to use Couchbase Lite Query’s Result Sets
Related Content — QueryBuilder | SQL++ for Mobile | Predictive Queries | Live Queries | Indexing
Query Execution
The execution of a Couchbase Lite for C’s database query returns an array of results, a result set.
Each row of the result set represents the data returned from a document that met the conditions defined by the WHERE statement of your query.
The composition of each row is determined by the SelectResult expressions provided in the SELECT statement.
Returned Results
The types of SelectResult formats you may encounter include those generated by :
-
QueryBuilder.select(SelectResult.all())— Using All -
QueryBuilder.select(SelectResult.expression(Meta.id))— Using Doc Id Metadata such as the_id -
QueryBuilder.select(SelectResult.property("myProp"))— Using Specific Properties
Return All Document Properties
The SelectResult returned by SelectResult.all() is a dictionary object, with the database name as the key and the document properties as an array of key-value pairs
[
{
"travel-sample": { (1)
"callsign": "MILE-AIR",
"country": "United States",
"iata": "Q5",
"icao": "MLA",
"id": 10,
"name": "40-Mile Air",
"type": "airline"
}
},
{
"travel-sample": { (2)
"callsign": "ALASKAN-AIR",
"country": "United States",
"iata": "AA",
"icao": "AAA",
"id": 10,
"name": "Alaskan Airways",
"type": "airline"
}
}
]Return Document Id Only
The SelectResult returned by queries using a SelectResult expression of the form SelectResult.expression(Meta.id) comprises a dictionary object with ID as the key and the ID value as the value.
[
{
"id": "hotel123"
},
{
"id": "hotel456"
},
]Return Specific Properties Only
The SelectResult returned by queries using one or more SelectResult expressions of the form SelectResult.expression(property("name")) ) comprises a key-value pair for each SelectResult expression in the query.
The key being the property name.
[
{ (1)
"id": "hotel123",
"type": "hotel",
"name": "Hotel Ghia"
},
{ (2)
"id": "hotel456",
"type": "hotel",
"name": "Hotel Deluxe",
}
]Processing Results
Access Document Properties - All Properties | Access Document Properties - ID | Access Document Properties - Selected Properties
To retrieve the results of your query, you need to execute it using Query.execute.
The output from the execution is an array, with each array element representing the data from a document that matched your search criteria.
To unpack the results you need to iterate through this array. Alternatively, you can convert the result to a JSON string — see:
Access Document Properties - All Properties
Here we look at how to access document properties when you have used SelectResult.all.
In this case each array element is a dictionary structure with the database name as its key. The properties are presented in the value as an array of key-value pairs (property name/property value).
You access the retrieved document properties by converting each row’s value, in turn, to a dictionary — as shown in Example 4.
CBLResultSet* results = CBLQuery_Execute(query, &err);
while(CBLResultSet_Next(results)) {
FLDict dict = FLValue_AsDict(CBLResultSet_ValueForKey(results, FLSTR("_")));
FLString id = FLValue_AsString(FLDict_Get(dict, FLSTR("id")));
FLString type = FLValue_AsString(FLDict_Get(dict, FLSTR("type")));
FLString name = FLValue_AsString(FLDict_Get(dict, FLSTR("name")));
FLString city = FLValue_AsString(FLDict_Get(dict, FLSTR("city")));
printf("ID :: %.*s\n", (int)id.size, (const char *)id.buf);
printf("Type :: %.*s\n", (int)type.size, (const char *)type.buf);
printf("Name :: %.*s\n", (int)name.size, (const char *)name.buf);
printf("City :: %.*s\n", (int)city.size, (const char *)city.buf);
}
// All results will be available from the above query
CBLResultSet_Release(results);| 1 | Here we get the dictionary of document properties using the database name as the key. You can add this dictionary to an array of returned matches, for processing elsewhere in the app. |
| 2 | Alternatively, you can access the document properties here, by using the property names as keys to the dictionary object. |
Access Document Properties - ID
Here we look at how to access document properties when you have returned only the document IDs for documents that matched your selection criteria.
This is something you may do when retrieval of the properties directly by the query may consume excessive amounts of memory and-or processing time.
In this case each array element is a dictionary structure where ID is the key and the required document ID is the value.
Access the required document properties by retrieving the document from the database using its document ID — as shown in Example 5.
// NOTE: No error handling, for brevity (see getting started)
CBLResultSet* results = CBLQuery_Execute(query, &err);
while(CBLResultSet_Next(results)) {
FLString id = FLValue_AsString(CBLResultSet_ValueForKey(results, FLSTR("id")));
printf("Document ID :: %.*s\n", (int)id.size, (const char *)id.buf);
}| 1 | Extract the Id value from the dictionary and use it to get the document from the database |
Access Document Properties - Selected Properties
Here we look at how to access properties when you have used SelectResult to get a specific subset of properties.
In this case each array element is an array of key value pairs (property name/property value).
Access the retrieved properties by converting each row into a dictionary — as shown in [ex-acc-specific].
// NOTE: No error handling, for brevity (see getting started)
CBLError err;
CBLQuery* query = CBLDatabase_CreateQuery(db, kCBLN1QLLanguage,
FLSTR("SELECT type, name, city FROM _"), NULL, &err);
CBLResultSet* results = CBLQuery_Execute(query, &err);
while(CBLResultSet_Next(results)) {
FLString type = FLValue_AsString(CBLResultSet_ValueForKey(results, FLSTR("type")));
FLString name = FLValue_AsString(CBLResultSet_ValueForKey(results, FLSTR("name")));
FLString city = FLValue_AsString(CBLResultSet_ValueForKey(results, FLSTR("city")));
printf("Type :: %.*s\n", (int)type.size, (const char *)type.buf);
printf("Name :: %.*s\n", (int)name.size, (const char *)name.buf);
printf("City :: %.*s\n", (int)city.size, (const char *)city.buf);
}JSON Result Sets
Use FLValue_ToJSON() to transform your result string into a JSON string, which can easily be serialized or used as required in your application. See <
CBLResultSet* results = CBLQuery_Execute(query, &err);
while(CBLResultSet_Next(results)) {
FLDict result = CBLResultSet_ResultDict(results);
FLStringResult json = FLValue_ToJSON((FLValue)result);
printf("JSON Result :: %.*s\n", (int)json.size, (const char *)json.buf);
FLSliceResult_Release(json);
}
CBLResultSet_Release(results);If your query selects ALL then the JSON format will be:
{
database-name: {
key1: "value1",
keyx: "valuex"
}
}If your query selects a sub-set of available properties then the JSON format will be:
{
key1: "value1",
keyx: "valuex"
}