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CRUD - Read - Java SDK
On this page
- Read Operations
- Read Characteristics
- Results Are Not Copies
- Results Are Lazy
- References Are Retained
- About the Examples on this Page
- Read from Realm
- Find a Specific Object by Primary Key
- Query All Objects of a Given Type
- Filter Queries Based on Object Properties
- Sort Query Results
- Query a Relationship
- Query an Inverse Relationship
- Aggregate Data
Read Operations
You can read back the data that you have stored in Realm. The standard data access pattern across Realm SDKs is to find, filter, and sort objects, in that order. To get the best performance from Realm as your app grows and your queries become more complex, design your app's data access patterns around a solid understanding of Realm read characteristics.
Read Characteristics
When you design your app's data access patterns around the following three key characteristics of reads in Realm, you can be confident you are reading data as efficiently as possible.
Results Are Not Copies
Results to a query are not copies of your data: modifying the results of a query will modify the data on disk directly. This memory mapping also means that results are live: that is, they always reflect the current state on disk.
Results Are Lazy
Realm defers execution of a query until you access the results. You can chain several filter and sort operations without requiring extra work to process the intermediate state.
References Are Retained
One benefit of Realm's object model is that Realm automatically retains all of an object's relationships as direct references, so you can traverse your graph of relationships directly through the results of a query.
A direct reference, or pointer, allows you to access a related object's properties directly through the reference.
Other databases typically copy objects from database storage into application memory when you need to work with them directly. Because application objects contain direct references, you are left with a choice: copy the object referred to by each direct reference out of the database in case it's needed, or just copy the foreign key for each object and query for the object with that key if it's accessed. If you choose to copy referenced objects into application memory, you can use up a lot of resources for objects that are never accessed, but if you choose to only copy the foreign key, referenced object lookups can cause your application to slow down.
Realm bypasses all of this using zero-copy live objects. Realm object accessors point directly into database storage using memory mapping, so there is no distinction between the objects in Realm and the results of your query in application memory. Because of this, you can traverse direct references across an entire realm from any query result.
About the Examples on this Page
The examples on this page use the data model of a project
management app that has two Realm object types: Project
and Task. A Project has zero or more Tasks.
See the schema for these two classes, Project and
Task, below:
Read from Realm
A read from a realm generally consists of the following steps:
Get all objects of a certain type from the realm.
Optionally, filter the results using the query engine.
Optionally, sort the results.
All query, filter, and sort operations return a results collection. The results collections are live, meaning they always contain the latest results of the associated query.
Important
Synchronous Reads and Writes on the UI Thread
By default, you can only read or write to a realm in your
application's UI thread using
asynchronous transactions. That is,
you can only use Realm methods whose name ends with the word
Async in the main thread of your Android application unless you
explicitly allow the use of synchronous methods.
This restriction exists for the benefit of your application users:
performing read and write operations on the UI thread can lead to
unresponsive or slow UI interactions, so it's usually best to handle
these operations either asynchronously or in a background thread.
However, if your application requires the use of synchronous
realm reads or writes on the UI thread, you can explicitly allow
the use of synchronous methods with the following
SyncConfiguration options:
Find a Specific Object by Primary Key
To find an object with a specific primary key value, open a realm and query the primary key field for the desired primary key value using the RealmQuery.equalTo() method:
Query All Objects of a Given Type
The first step of any read is to get all objects of a certain type in a realm. With this results collection, you can operate on all instances on a type or filter and sort to refine the results.
In order to access all instances of ProjectTask and Project, use
the where() method
to specify a class:
Filter Queries Based on Object Properties
A filter selects a subset of results based on the value(s) of one or more object properties. Realm provides a full-featured query engine you can use to define filters. The most common use case is to find objects where a certain property matches a certain value. Additionally, you can compare strings, aggregate over collections of numbers, and use logical operators to build up complex queries.
In the following example, we use the query engine's comparison operators to:
Find high priority tasks by comparing the value of the
priorityproperty value with a threshold number, above which priority can be considered high.Find just-started or short-running tasks by seeing if the
progressMinutesproperty falls within a certain range.Find unassigned tasks by finding tasks where the
assigneeproperty is equal to null.Find tasks assigned to specific teammates Ali or Jamie by seeing if the
assigneeproperty is in a list of names.
Sort Query Results
A sort operation allows you to configure the order in which Realm returns queried objects. You can sort based on one or more properties of the objects in the results collection.
Realm only guarantees a consistent order of results when the results are sorted.
The following code sorts the projects by name in reverse alphabetical order (i.e. "descending" order).