Table of Contents

Class AssociationAttribute

Namespace
LinqToDB.Mapping
Assembly
linq2db.dll

Defines relation between tables or views. Could be applied to:

  • instance properties and fields;
  • instance and static methods.

For associations, defined using static methods, this mapping side defined by type of first parameter. Also, optionally, you can pass data context object as extra method parameter.

Based on association type - to one or to multiple records - result type should be target record's mapping type or IEquatable<T> collection.

By default associations are used only for joins generation in LINQ queries and will have null value for loaded records. To load data into association, you should explicitly specify it in your query using LoadWith<TEntity, TProperty>(IQueryable<TEntity>, Expression<Func<TEntity, TProperty?>>) method.

[AttributeUsage(AttributeTargets.Method|AttributeTargets.Property|AttributeTargets.Field, AllowMultiple = false)]
public class AssociationAttribute : MappingAttribute, _Attribute
Inheritance
AssociationAttribute
Implements
Inherited Members
Extension Methods

Constructors

AssociationAttribute()

Creates attribute instance.

public AssociationAttribute()

Properties

AliasName

Gets or sets alias for association. Used in SQL generation process.

public string? AliasName { get; set; }

Property Value

string

AssociationSetterExpression

Specifies a setter expression. If is set, it will be used to set the storage member when using LoadWith(). Action takes two parameters: the storage member and the value to assign to it.

Expression<Action<SomeContainerType,SomeOtherEntity>> setContainerValue;

        setContainerValue = (container, value) => container.Value = value;</code></pre></example></p>




public Expression? AssociationSetterExpression { get; set; }

Property Value

Expression

AssociationSetterExpressionMethod

Specifies static property or method without parameters, that returns a setter expression. If is set, it will be used to set the storage member when using LoadWith(). Result of method should be Action which takes two parameters: the storage member and the value to assign to it.

public class SomeEntity
            {
                [Association(SetExpressionMethod = nameof(OtherImpl), CanBeNull = true)]
                public SomeOtherEntity Other { get; set; }

            public static Expression<Action<SomeContainerType,SomeOtherEntity>> OtherImpl()
            {
                return (container, value) => container.Value = value;
            }
        }</code></pre></example></p>




public string? AssociationSetterExpressionMethod { get; set; }

Property Value

string

CanBeNull

Defines type of join:

  • inner join for CanBeNull = false;
  • outer join for CanBeNull = true. When using Configuration.UseNullableTypesMetadata, the default value for associations (cardinality 1) is derived from nullability. Otherwise the default value is true (for collections and when option is disabled).
public bool CanBeNull { get; set; }

Property Value

bool

ExpressionPredicate

Specifies static property or method without parameters, that returns join predicate expression. This predicate will be used together with ThisKey/OtherKey join keys, if they are specified. Predicate expression lambda function takes two parameters: this record and other record and returns boolean result.

public string? ExpressionPredicate { get; set; }

Property Value

string

OtherKey

Gets or sets comma-separated list of association key members on another side of association. Those keys will be used for join predicate generation and must be compatible with ThisKey keys. You must specify keys it you do not use custom predicate (see ExpressionPredicate).

public string? OtherKey { get; set; }

Property Value

string

Predicate

Specifies predicate expression. This predicate will be used together with ThisKey/OtherKey join keys, if they are specified. Predicate expression lambda function takes two parameters: this record and other record and returns boolean result.

public Expression? Predicate { get; set; }

Property Value

Expression

QueryExpression

Specifies query expression. If is set, other association keys are ignored. Lambda function takes two parameters: this record, IDataContext and returns IQueryable result.

Expression<Func<SomeEntity, IDataContext, IQueryable<SomeOtherEntity>>> associationQuery;

        associationQuery = (e, db) => db.GetTable<SomeOtherEntity>().Where(se => se.Id == e.Id);</code></pre></example></p>




public Expression? QueryExpression { get; set; }

Property Value

Expression

QueryExpressionMethod

Specifies static property or method without parameters, that returns IQueryable expression. If is set, other association keys are ignored. Result of query method should be lambda which takes two parameters: this record, IDataContext and returns IQueryable result.

public class SomeEntity
            {
                [Association(ExpressionQueryMethod = nameof(OtherImpl), CanBeNull = true)]
                public SomeOtherEntity Other { get; set; }

            public static Expression<Func<SomeEntity, IDataContext, IQueryable<SomeOtherEntity>>> OtherImpl()
            {
                return (e, db) => db.GetTable<SomeOtherEntity>().Where(se => se.Id == e.Id);
            }
        }</code></pre></example></p>




public string? QueryExpressionMethod { get; set; }

Property Value

string

Storage

Specify name of property or field to store association value, loaded using LoadWith<TEntity, TProperty>(IQueryable<TEntity>, Expression<Func<TEntity, TProperty?>>) method. When not specified, current association member will be used.

public string? Storage { get; set; }

Property Value

string

ThisKey

Gets or sets comma-separated list of association key members on this side of association. Those keys will be used for join predicate generation and must be compatible with OtherKey keys. You must specify keys it you do not use custom predicate (see ExpressionPredicate).

public string? ThisKey { get; set; }

Property Value

string

Methods

GetObjectID()

Returns mapping attribute id, based on all attribute options.

public override string GetObjectID()

Returns

string

GetOtherKeys()

Returns OtherKey value as a list of key member names.

public string[] GetOtherKeys()

Returns

string[]

List of key members.

GetThisKeys()

Returns ThisKey value as a list of key member names.

public string[] GetThisKeys()

Returns

string[]

List of key members.