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Query Engine - .NET SDK

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  • Operators
  • Comparison Operators
  • Logical Operators
  • String Operators
  • Aggregate Operators

To filter data in your realm, you can leverage Realm Database's query engine. Realm Database's query engine implements standard LINQ syntax. See the exact scope of LINQ implemented on the LINQ Support page.

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 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
  • A value of any type used by realm (string, date, number, boolean, etc.)
Note
About the examples on this page

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:

public class Task : RealmObject
{
[PrimaryKey]
[MapTo("_id")]
public ObjectId Id { get; set; } = ObjectId.GenerateNewId();
public string Name { get; set; }
public string Assignee { get; set; }
public bool IsComplete { get; set; }
public int Priority { get; set; }
public int ProgressMinutes { get; set; }
}
public class Project : RealmObject
{
[PrimaryKey]
[MapTo("_id")]
public ObjectId ID { get; set; } = ObjectId.GenerateNewId();
public string Name { get; set; }
public IList<Task> Tasks { get; }
}

Value comparisons

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.
<
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.
Example

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 to null.
  • Find tasks assigned to specific teammates Ali or Jamie by seeing if the assignee property is in a list of names.
var highPri = tasks.Where(t => t.Priority > 5);
var quickTasks = tasks.Where(t =>
t.ProgressMinutes >= 1 &&
t.ProgressMinutes < 15);
var unassignedTasks = tasks.Where(t =>
t.Assignee == null);
var AliOrJamieTasks = tasks.Where(t =>
t.Assignee == "Ali" ||
t.Assignee == "Jamie");

You can use the logical operators listed in the following table to make compound predicates:

Operator
Description
&&
Evaluates to true if both left-hand and right-hand expressions are true.
!
Negates the result of the given expression.
||
Evaluates to true if either expression returns true.
Example

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:

var completedTasksForAli = tasks.Where(t => t.Assignee == "Ali"
&& t.IsComplete);

You can compare string values using the string operators listed in the following table. Regex-like wildcards allow more flexibility in search.

Operator
Description
StartsWith
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.
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 true if the left-hand string expression matches the right-hand string wildcard string expression. A wildcard string expression is a string that uses normal characters with two special wildcard characters:

  • The * wildcard matches zero or more of any character
  • The ? wildcard matches any character.

For example, the wildcard string "d?g" matches "dog", "dig", and "dug", but not "ding", "dg", or "a dog".

Equals
Evaluates to true if the left-hand string is lexicographically equal to the right-hand string.
Contains
Evaluates to true if the left-hand string expression is found anywhere in the right-hand string expression.
string.IsNullOrEmpty
Evaluates to true if the left-hand string expression is null or empty. Note that IsNullOrEmpty() is a static method on string.
Example

The following examples use the query engine's string operators to find tasks:

// Note: In each of the following examples, you can replace the
// Where() method with First(), FirstOrDefault(),
// Single(), SingleOrDefault(),
// Last(), or LastOrDefault().
// Get all tasks where the Assignee's name starts with "E" or "e"
var tasksStartWitE = tasks.Where(t => t.Assignee.StartsWith("E",
StringComparison.OrdinalIgnoreCase));
// Get all tasks where the Assignee's name ends wth "is"
// (lower case only)
var endsWith = tasks.Where(t =>
t.Assignee.EndsWith("is", StringComparison.Ordinal));
// Get all tasks where the Assignee's name contains the
// letters "ami" in any casing
var tasksContains = tasks.Where(t => t.Assignee.Contains("ami",
StringComparison.OrdinalIgnoreCase));
// Get all tasks that have no assignee
var null_or_empty = tasks.Where(t => string.IsNullOrEmpty(t.Assignee));
Important
Case Comparisons

When evaluating strings, the second parameter in all functions except Like must be either StringComparison.OrdinalIgnoreCase or StringComparison.Ordinal. For the Like() method, the second parameter is a boolean value (where "true" means "case sensitive").

Aggregate operators traverse a collection and reduce it to a single value. Note that aggregations use the Filter() method, which can be used to create more complex queries that are currently unsupported by the LINQ provider. Filter() supports SORT and DISTINCT clauses in addition to filtering.

You can apply any of the following aggregate operators:

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. This is currently only supported on to-many relationship collections and not on lists of primitives. In order to use @count on a list of primitives, consider wrapping the primitives in a Realm object.
@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.
Example

The following examples show different ways to aggregate data:

// Get all projects with an average Task priorty > 5:
var avgPriority = projects.Filter(
"Tasks.@avg.Priority > 5");
// Get all projects where all Tasks are high-priority:
var highPriProjects = projects.Filter(
"Tasks.@min.Priority > 5");
// Get all projects with long-running Tasks:
var longRunningProjects = projects.Filter(
"Tasks.@sum.ProgressMinutes > 100");
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