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Showing content from https://www.mongodb.com/docs/atlas/device-sdks/sdk/dotnet/model-data/define-object-model/ below:

Object Models - .NET SDK - Atlas Device SDKs

Realm classes are regular C# classes that define the Realm schema.

All Realm objects inherit from the IRealmObject, IEmbeddedObject, or IAsymmetricObject interface and must be declared partial classes.

In versions of the .NET SDK older than 10.18.0, objects derive from RealmObject, EmbeddedObject, or AsymmetricObject base classes. This approach to Realm model definition is still supported, but does not include new features such as the nullability annotations. In a future SDK release, the base classes will become deprecated. You should use the interfaces for any new classes that you write and should consider migrating your existing classes.

Class names are limited to a maximum of 57 UTF-8 characters.

An object schema is a configuration object that defines the properties and relationships of a Realm object. Realm client applications define object schemas with the native class implementation in their respective language using the Object Schema.

Object schemas specify constraints on object properties such as the data type of each property and whether or not a property is required. Schemas can also define relationships between object types in a realm.

Every App has a App Services Schema composed of a list of object schemas for each type of object that the realms in that application may contain. Realm guarantees that all objects in a realm conform to the schema for their object type and validates objects whenever they're created, modified, or deleted.

Schema properties are standard C# properties on a RealmObject. There are several property annotations that you can use to more finely define how a Realm handles a specific property.

A primary key is a property that uniquely identifies an object. You can create a primary key with any of the following types (or their nullable counterparts):

• ObjectId

• UUID

• string

• char

• byte

• short

• int

• long

You may define a primary key on a single property for an object type as part of the object schema. Realm automatically indexes primary key properties, which allows you to efficiently read and modify objects based on their primary key.

If an object type has a primary key, then all objects of that type must include the primary key property with a value that is unique among objects of the same type in a realm.

Once you assign a property as a primary key, you cannot change it.

The following example demonstrates how to designate a primary key in an object schema:

public partial class Dog : IRealmObject{    [PrimaryKey]    public string Name { get; set; }    public int Age { get; set; }    public Person? Owner { get; set; }}

Indexes significantly improve query times in a Realm. Without indexes, Realm scans every document in a collection to select the documents that match the given query. However, if an applicable index exists for a query, Realm uses the index to limit the number of documents that it must inspect.

You can index properties with the following types:

• bool

• byte

• short

• int

• long

• DateTimeOffset

• char

• string

• ObjectId

• UUID

Adding an index speeds up queries at the cost of slightly slower write times and additional storage and memory overhead. Indexes require space in your realm file, so adding an index to a property increases disk space consumed by your realm file. Each index entry is a minimum of 12 bytes.

To index a property, use the Indexed attribute. With the Indexed attribute, you can specify the type of index on the property by using the IndexType enum. In the following example, we have a default ("General") index on the Name property:

public partial class Person : IRealmObject{    [Indexed(IndexType.General)]    public string Name { get; set; }    [Indexed(IndexType.FullText)]    public string Biography { get; set; }}

When you create an index, you are creating it on the local realm and not on an Atlas collection. If you need to query an Atlas collection directly and want to improve performance, refer to Create, View, Drop, and Hide Indexes.

In addition to standard indexes, Realm also supports Full-Text Search (FTS) indexes on string properties. While you can query a string field with or without a standard index, an FTS index enables searching for multiple words and phrases and excluding others.

For more information on querying full-text indexes, see Full Text Search (LINQ) and Full Text Search (RQL).

To index an FTS property, use the Indexed attribute with the IndexType.FullText enum. In the following example, we have a FullText index on the Biography property:

public partial class Person : IRealmObject{    [Indexed(IndexType.General)]    public string Name { get; set; }    [Indexed(IndexType.FullText)]    public string Biography { get; set; }}

You can use the built-in language features to assign a default value to a property. In C#, you can assign a default value on primitives in the property declaration. You cannot set a default value on a collection, except to set it to null!. Even if you set a collection to null!, collections are always initialized on first access, so will never be null.

public partial class Person : IRealmObject{    public string Name { get; set; } = "foo";    public IList<PhoneNumber> PhoneNumbers { get; } = null!;}

While default values ensure that a newly created object cannot contain a value of null (unless you specify a default value of null), they do not impact the nullability of a property. To make a property non-nullable, see Required Properties.

If you don't want to save a property in your model to a realm, you can ignore that property. A property is ignored by default if it is not autoimplemented or does not have a setter.

Ignore a property from a Realm object model with the Ignored attribute:

public string ThumbnailPath { get; set; }[Ignored]public Image? Thumbnail { get; set; }

By default, Realm uses the name defined in the model class to represent properties internally. In some cases you might want to change this behavior:

• To make it easier to work across platforms, since naming conventions differ.

• To change a property name in .NET without forcing a migration.

Choosing an internal name that differs from the name used in model classes has the following implications:

• Migrations must use the internal name when creating classes and properties.

• Schema errors reported will use the internal name.

Use the [MapTo] attribute to rename a property:

public partial class Person : IRealmObject{    [MapTo("moniker")]    public string Name { get; set; }}

By default, Realm uses the name defined in the model class to represent classes internally. In some cases you might want to change this behavior:

• To support multiple model classes with the same simple name in different namespaces.

• To make it easier to work across platforms, since naming conventions differ.

• To use a class name that is longer than the 57 character limit enforced by Realm.

• To change a class name in .NET without forcing a migration.

Use the [MapTo] attribute to rename a class:

[MapTo("Human")]public partial class Person : IRealmObject{    public string Name { get; set; }}

Realm will not store a property with a custom setter. To use a custom setter, store the property value in a private property and then map that value to a public property with the custom setter. Realm will store the private property, while you modify its value via the public property. In the following code, the private email property is stored in the realm, but the public Email property, which provides validation, is not persisted:

private string email { get; set; }public string Email{    get { return email; }    set    {        if (!value.Contains("@")) throw new Exception("Invalid email address");        email = value;    }}

New in version 12.2.0.

Starting in SDK version 12.2.0, you can store collections of mixed data within a RealmValue property. You can use this feature to model complex data structures, such as JSON or MongoDB documents, without having to define a strict data model.

Unstructured data is data that doesn't easily conform to an expected schema, making it difficult or impractical to model to individual data classes. For example, your app might have highly variable data or dynamic data whose structure is unknown at runtime.

Storing collections in a mixed property offers flexibility without sacrificing functionality, including performant synchronization when using Device Sync. And you can work with them the same way you would a non-mixed collection:

• You can nest mixed collections up to 100 levels.

• You can query on and react to changes on mixed collections.

• You can find and update individual mixed collection elements.

However, storing data in mixed collections is less performant than using a structured schema or serializing JSON blobs into a single string property.

To model unstructured data in your app, define the appropriate properties in your schema as RealmValue types. You can then set these RealmValue properties as a list or a dictionary of RealmValue elements. Note that RealmValue cannot represent a set or an embedded object.

• Use a map of mixed data types when the type is unknown but each value will have a unique identifier.

• Use a list of mixed data types when the type is unknown but the order of objects is meaningful.

By default, your application's Realm Schema includes all classes that implement IRealmObject or IEmbeddedObject. If you only want to include a subset of these classes in your Realm Schema, you can update your configuration to include the specific classes you want:

partial class LoneClass : IRealmObject{    public string Name { get; set; }}class AnotherClass{    private void SetUpMyRealmConfig()    {                var config = new RealmConfiguration("RealmWithOneClass.realm");        config.Schema = new[] { typeof(LoneClass) };                config.Schema = new[] { typeof(Dog), typeof(Cat) };    }}

In C#, value types, such as int and bool, are implicitly non-nullable. However, they can be made optional by using the question mark (?) notation.

Beginning with C# 8.0, nullable reference types were introduced. If your project is using C# 8.0 or later, you can also declare reference types, such as string and byte[], as nullable with ?.

The Realm .NET SDK fully supports the nullable-aware context and uses nullability to determine whether a property is required or optional. The SDK has the following rules:

• Realm assumes that both value- and reference-type properties are required if you do not designate them as nullable. If you designate them as nullable by using ?, Realm considers them optional.

• You must declare properties that are Realm object types as nullable.

• You cannot declare collections (list, sets, backlinks, and dictionaries) as nullable, but their parameters may be nullable according to the following rules:

  • For all types of collections, if the parameters are primitives (value- or reference-types), they can be required or nullable.

  • For lists, sets, and backlinks, if the parameters are Realm objects, they cannot be nullable.

  • For dictionaries with a value type of Realm object, you must declare the value type parameter as nullable.

The following code snippet demonstrates these rules:

#nullable enablepublic partial class Person : IRealmObject{        public string NonNullableName { get; set; }    public string? NullableName { get; set; }    public byte[] NonNullableArray { get; set; }    public byte[]? NullableArray { get; set; }        public int NonNullableInt { get; set; }    public int? NullableInt { get; set; }        public Dog? NullableDog { get; set; }            public IList<int> IntListWithNonNullableValues { get; }    public IList<int?> IntListWithNullableValues { get; }            public IList<Dog> ListOfNonNullableObjects { get; }            public ISet<Dog> SetOfNonNullableObjects { get; }            public IDictionary<string, Dog?> DictionaryOfNullableObjects { get; }            [Backlink(nameof(Dog.Person))]    public IQueryable<Dog> MyDogs { get; }        }

If you are using the older schema type definition (your classes derive from the RealmObject base class), or you do not have nullability enabled, you will need to use the [Required] attribute for any required string and byte[] property.

You may prefer to have more flexibility in defining the nullability of properties in your Realm objects. You can do so by setting realm.ignore_objects_nullability = true in a global configuration file.

If you enable realm.ignore_objects_nullability, nullability annotations will be ignored on Realm object properties, including collections of Realm objects.

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