如何使用 EntityFramework Core 在两个模块之间创建关系？

Question

I need to build a relation between two tables without using the primary key property.

Here are my entity models

public class RealEstateProperty
{
    public int Id { get; set; }
    public string PostalCode { get; set; }
    //... List of all properties

    [ForeignKey(nameof(PostalCode))]
    public virtual RealEstatePropertyPostalCodePriority PostalCodePriority { get; set; }
}

public class RealEstatePropertyPostalCodePriority
{
    public int Id { get; set; }

    // This is a unique property on the database
    public string PostalCode { get; set; }
    public int? Sort { get; set; }

    [ForeignKey(nameof(PostalCode)), InverseProperty(nameof(RealEstateProperty.PostalCodePriority))]
    public ICollection<RealEstateProperty> Properties { get; set; }
}

The above relations throw the following exception

InvalidOperationException: The relationship from 'RealEstateProperty.PostalCodePriority' to 'RealEstatePropertyPostalCodePriority.Properties' with foreign key properties {'PostalCode' : string} cannot target the primary key {'Id' : int} because it is not compatible. Configure a principal key or a set of compatible foreign key properties for this relationship.

The end result will need to look something like this

SELECT p.Id, p.PostalCode, z.Sort
FROM RealEstateProperties AS p
LEFT JOIN RealEstatePropertyPostalCodePriorities AS z ON z.PostalCode = p.PostalCode

Answer 1

In order to be used as principal end of a relationship, the non PK property PostalCode of the RealEstatePropertyPostalCodePriority must be configured as alternate key . Then the relationship must be configured to use the alternate key rather than the primary key.

Both require fluent API ( HasAlternateKey , HasPrincipalKey ), so remove the ForeignKey and InverveProperty annotations (they are confusing anyway):

public class RealEstateProperty
{
    public int Id { get; set; }
    public string PostalCode { get; set; }
    //... List of all properties

    //[ForeignKey(nameof(PostalCode))] <-- remove this...
    public virtual RealEstatePropertyPostalCodePriority PostalCodePriority { get; set; }
}

public class RealEstatePropertyPostalCodePriority
{
    public int Id { get; set; }

    // This is a unique property on the database
    public string PostalCode { get; set; }
    public int? Sort { get; set; }

    //[ForeignKey(nameof(PostalCode)), InverseProperty(nameof(RealEstateProperty.PostalCodePriority))] // <-- ... and this
    public ICollection<RealEstateProperty> Properties { get; set; }
}

and use the following fluent configuration instead:

modelBuilder.Entity<RealEstatePropertyPostalCodePriority>()
    .HasMany(pcp => pcp.Properties)
    .WithOne(p => p.PostalCodePriority)
    .HasForeignKey(p => p.PostalCode)
    .HasPrincipalKey(pcp => pcp.PostalCode);