Live Queries - iOS SDK¶
To get the best performance from MongoDB Realm as your app grows and your queries become more complex, design your app's data access patterns around a solid understanding of Realm Database read characteristics.
To learn how to read and write data, see Read & Write Data - iOS SDK.
All Realm objects are live objects, which means they automatically update whenever they're modified. Realm emits a notification event whenever any property changes.
You can use live objects to work with object-oriented data natively without an ORM tool. Live objects are direct proxies to the underlying stored data, which means that a live object doesn't directly contain data. Instead, a live object always references the most up-to-date data on disk and lazy loads property values when you access them from a collection. This means that a realm can contain many objects but only pay the performance cost for data that the application is actually using.
Valid write operations on a live object automatically persist to the realm and propagate to any other synced clients. You do not need to call an update method, modify the realm, or otherwise "push" updates.
Read from Realm Database¶
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.
- 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.
When you design your app's data access patterns around the following three key characteristics of reads in Realm Database, 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.
See also: Collections are Live.
Results Are Lazy¶
Realm Database 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.
See also: Results are Lazily Evaluated.
References Are Retained¶
One benefit of Realm Database's object model is that Realm Database 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 Database 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 Database 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.