More About Queries

Basic queries

We have shown how to fetch individual records by ids, but in real-world application there are usually needs to show a list of items according to some criteria. It is supported by queries in Skygear.

Let's see how to fetch a list of to-do items to be displayed in our hypothetical To-Do app:

SKYQuery *query = [SKYQuery queryWithRecordType:@"todo" predicate:nil];

NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"order" ascending:YES];
query.sortDescriptors = @[sortDescriptor];

SKYDatabase *privateDB = [[SKYContainer defaultContainer] privateCloudDatabase];
[privateDB performQuery:query completionHandler:^(NSArray *results, NSError *error) {
    if (error) {
        NSLog(@"error querying todos: %@", error);
        return;
    }

    NSLog(@"Received %@ todos.", @(results.count));
    for (SKYRecord *todo in results) {
        NSLog(@"Got a todo: %@", todo[@"title"]);
    }
}];

let query = SKYQuery(recordType: "todo", predicate: nil)
let sortDescriptor = NSSortDescriptor(key: "order", ascending: true)
query.sortDescriptors = [sortDescriptor]

SKYContainer.default().privateCloudDatabase.perform(query) { (results, error) in
    if error != nil {
        print ("error querying todos: \(error)")
        return
    }

    print ("Received \(results?.count) todos.")
    for todo in results as! [SKYRecord] {
        print ("Got a todo \(todo["title"])")
    }
}

We constructed a SKYQuery to search for todo records. There are no additional criteria needed so we put the predicate to nil. Then we assigned a NSSortDescription to ask Skygear Server to sort the todo records by order field in ascending order.

Query conditions

To use SKYQuery with ease, we recommend using the methods provided to add constraints. However, you can also use NSPredicate to add constraints if you wish. The following features are supported:

Basic Comparisons

Strings

IN

The IN operator can be used to query a key for value that matches one of the item in an NSArray.

NSPredicate *inPredicate =
            [NSPredicate predicateWithFormat: @"attribute IN %@", aCollection];

let inPredicate = NSPredicate(format: "attribute IN %@", aCollection)

If the key being queried is a JSON type, the IN operator can also be used to query the key to check if it contains a particular value:

NSPredicate *inPredicate =
            [NSPredicate predicateWithFormat: @"%@ IN attribute", aValue];

let inPredicate = NSPredicate(format: "%@ IN attribute", aValue)

Social Relation Predicate

The SKYRelationPredicate can be used to query for records having a relation with the current user. For this kind of query, the record have an relation with the current user if the record has an attribute that contains a user having the relation with the current user.

For example, to query for records owned by a user that the current user is following:

NSPredicate *p =
            [SKYRelationPredicate predicateWithRelation:[SKYRelation followingRelation]
                                                    key:@"_owner"]

let p = SKYRelationPredicate(relation: SKYRelation.following(), keyPath: "_owner")

Pagination and ordering

Sorting the records

We can sort records returned by:

SKYQuery *query = [SKYQuery queryWithRecordType:@"order" predicate:nil];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"_updated_at" ascending:NO];     // sorted by modificationDate
query.sortDescriptors = @[sortDescriptor];     // apply the NSSortDescriptor to the query

let query = SKYQuery(recordType: "order", predicate: nil)
let sortDescriptor = NSSortDescriptor(key: "_updated_at", ascending: false)     // sorted by modificationDate
query.sortDescriptors = [sortDescriptor]     // apply the NSSortDescriptor to the query

SKYQuery utilizes NSPredicate to apply filtering on query results. You can use other parameters to sort your queries.

Limiting and Offset

We can limit the numbers of records returned by:

query.limit = 10;     // only show the top 10 records

query.limit = 10	  // only show the top 10 records

We can also set an offset number to the query by:

query.offset = 5;     // ignore the first 5 records

query.offset = 5	  // ignore the first 5 records

Setting an offset number means skipping that many rolls before beginning to return rows. If the offset number is 0, then no rows will be skipped. If you use both limit and offset, then offset numbers of rows will be skipped before starting to limit the number of rows returned.

Now the first 5 records in the result list are skipped. The query result starts with the 6th record. It works just like SQL offset.

Record counts

To get the number of all records matching a query, set the property overallCount property of SKYQuery to YES. The record count can be retrieved from overallCount property of SKYQueryOperation when perRecordCompletionBlock is first called.

Getting the reserved columns

For each record type stored in the database, a table with the same name as the record type is created. For example, if your record type is called note, there is a table called note in the database. Each row in the table corresponds to one record.

For each record table there exists two types of columns, those that are reserved by Skygear and those that are user-defined. Reserved columns contain metadata of a record, such as record ID, record owner and creation time. Names of reserved columns are prefixed with underscore (_).

It is possible to manipulate data in record tables directly. However, one should exercise cautions when modifying data directly in record tables.

Each record table contains the following reserved columns:

You can retrieve the values from the object by accessing its properties:

NSDate *creationDate = [noteObject creationDate];
NSString *creatorID = [noteObject creatorUserRecordID];
SKYRecordID *recordID = [record recordID];
NSString *recordType = [record recordType];

let creationDate = noteObject.creationDate
let creatorID = noteObject.creatorUserRecordID
let record = record.recordID
let recordType = record.recordType

Please head to Database Schema to read more about Reserved Columns, Record Tables and Reserved Tables.