Query Engine - React Native SDK
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Overview
To filter data in your Realms, construct queries with Realm Query Language. The Realm query engine processes these queries.
Refer to the Realm Query Language Reference for more information about syntax, usage and limitations.
Operators
There are several types of operators available to filter a
Realm collection.
Filters work by evaluating an operator expression for
every object in the collection being
filtered. If the expression resolves to true
, Realm
Database includes the object in the results collection.
An expression consists of one of the following:
- The name of a property of the object currently being evaluated.
- An operator and up to two argument expression(s).
- A literal string, number, or date.
The examples in this page use a simple data set for a
task list app. The two Realm object types are Project
and Task
. A Task
has a name, assignee's name, and
completed flag. There is also an arbitrary number for
priority -- higher is more important -- and a count of
minutes spent working on it. A Project
has zero or more
Tasks
.
See the schema for these two classes, Project
and
Task
, below:
Comparison Operators
The most straightforward operation in a search is to compare values.
Operator | Description |
---|---|
== | Evaluates to true if the left-hand expression is equal to the right-hand expression. |
> | Evaluates to true if the left-hand numerical or date expression is greater than the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is later than the right-hand date. |
>= | Evaluates to true if the left-hand numerical or date expression is greater than or equal to the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is later than or the same as the right-hand date. |
in | Evaluates to true if the left-hand expression is in the right-hand list. |
< | Evaluates to true if the left-hand numerical or date expression is less than the right-hand numerical or date expression. For dates, this evaluates to true if the left-hand date is earlier than the right-hand date. |
<= | Evaluates to true if the left-hand numeric expression is less than or equal to the right-hand numeric expression. For dates, this evaluates to true if the left-hand date is earlier than or the same as the right-hand date. |
!= | Evaluates to true if the left-hand expression is not equal to the right-hand expression. |
The following example uses the query engine's comparison operators to:
- Find high priority tasks by comparing the value of the
priority
property value with a threshold number, above which priority can be considered high. - Find just-started or short-running tasks by seeing if the
progressMinutes
property falls within a certain range. - Find unassigned tasks by finding tasks where the
assignee
property is equal tonull
. - Find tasks assigned to specific teammates Ali or Jamie by seeing if the
assignee
property is in a list of names.
Logical Operators
You can make compound predicates using logical operators.
Operator | Description |
---|---|
and && | Evaluates to true if both left-hand and right-hand expressions are true . |
not ! | Negates the result of the given expression. |
or || | Evaluates to true if either expression returns true . |
We can use the query language's logical operators to find
all of Ali's completed tasks. That is, we find all tasks
where the assignee
property value is equal to 'Ali' AND
the isComplete
property value is true
:
String Operators
You can compare string values using these string operators. Regex-like wildcards allow more flexibility in search.
Operator | Description |
---|---|
beginsWith | Evaluates to true if the left-hand string expression begins with the right-hand string expression. This is similar to contains , but only matches if the left-hand string expression is found at the beginning of the right-hand string expression. |
contains | Evaluates to true if the left-hand string expression is found anywhere in the right-hand string expression.
Not supported on the C# SDK. |
endsWith | Evaluates to true if the left-hand string expression ends with the right-hand string expression. This is similar to contains , but only matches if the left-hand string expression is found at the very end of the right-hand string expression. |
like | Evaluates to
For example, the wildcard string "d?g" matches "dog", "dig", and "dug", but not "ding", "dg", or "a dog". |
== (JS, Obj-C, Swift) | Evaluates to true if the left-hand string is lexicographically equal to the right-hand string. |
We use the query engine's string operators to find projects with a name starting with the letter 'e' and projects with names that contain 'ie':
Aggregate Operators
You can apply an aggregate operator to a collection property of a Realm object. Aggregate operators traverse a collection and reduce it to a single value.
Operator | Description |
---|---|
@avg | Evaluates to the average value of a given numerical property across a collection. |
@count | Evaluates to the number of objects in the given collection. |
@max | Evaluates to the highest value of a given numerical property across a collection. |
@min | Evaluates to the lowest value of a given numerical property across a collection. |
@sum | Evaluates to the sum of a given numerical property across a collection. |
We create a couple of filters to show different facets of the data:
- Projects with average tasks priority above 5.
- Long running projects.
Set Operators
A set operator uses specific rules to determine whether to pass each input collection object to the output collection by applying a given predicate to every element of a given list property of the object.
Operator | Description |
---|---|
All | Returns objects where the predicate evaluates to true for all objects in the collection. |
Any | Returns objects where the predicate evaluates to true for any objects in the collection. |
None | Returns objects where the predicate evaluates to false for all objects in the collection. |
We use the query engine's set operators to find:
- Projects with no complete tasks.
- Projects with any top priority tasks.
Summary
- There are several categories of operators available to filter results: comparison, logical, string, aggregate, and set operators.