More About 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:
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.
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:
IN operator can be used to query a key for value that matches one of the
item in an
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:
Social Relation Predicate
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:
Pagination and ordering
Sorting the records
We can sort records returned by:
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:
We can also set an offset number to the query by:
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
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.
To get the number of all records matching a query, set the property
overallCount property of
YES. The record count can be
overallCount property of
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:
|Column Name||Object Attribute||Description|
You can retrieve the values from the object by accessing its properties:
Please head to Database Schema to read more about Reserved Columns, Record Tables and Reserved Tables.