当两个表中都不存在ID时，使用Linq连接实体框架表

Question

Below is a class I have used to generate a table in my database using Entity Framework. I'd like to be able to link this table to another table, Property . However, the way my code is set up there is not an Id column in the Instruction table, there is a Property property within the class, which then generates a PropertyId column in the actual database, but since the Property property is not an Id I am unable to using Linq to join these tables.

Instruction table

[Table("Instruction")]
public class Instruction
    {
        [Key]
        public int Id { get; set; }
        public InstructionTypes InstructionType { get; set; }
        public Property Property { get; set; } //Generates the EF property FK, but is not an ID so therefore cannot be used in linq.
    }

Property table

[Table("Property")]
    public partial class Property
    {
        [Key]
        public int Id { get; set; }
        public Address Correspondence { get; set; }
    }

Join Query

var instruction = 
                from instructions in _context.Instructions
                join properties in _context.Properties on instructions.Property equals properties.Id
                where ...

The above query gives a compiler error of: `The type of one of the expressions in the join clause is incorrect.

This error is being generated as I'm attempting to use a property object to join with a propertyId.

How can I alter this query so that I am able to join these two tables?

Answer 1

You seems to be a newcomer to linq. As such you are still thinking as if you still are in an sql world.

With linq to entities, the use of join is the exception. SQL join are generated silently by EF using the navigation properties.

So your query can be:

var instruction = 
            from instruction in _context.Instructions                
            where instruction.Porperty.Correspondence.Contains("abc");

then you can access

instruction.First().Property.Correspondence

As a good practice you can delclare the foreign keys as class members and use the fluent API to bind them.

To test you can use the following code,

//assuming that Instructions is a DbSet<Instruction>
using (var context = new MyContext() ) {
    context.Instructions.Add(
        new instruction {
            Property = new Property {
                Correspondence = new Address {}
            }
        });
}

using (var context = new MyContext() ) {
    var c = context.Instructions.First();
    console.WriteLine($"{c.Id}, {c?.Property.Id}, {c?.Property?.Correspondence.Id}");
});

Answer 2

In 99% of all cases, you do not want to use the join operator. Entity Framework automatically generates SQL JOINS for you when you are using Navigation Properties.

var instruction = await _context.Instructions.Where(i => i.Property...).FirstOrDefaultAsync().ConfigureAwait(false);

Note, that depending on whether you are using EF6 or EF Core or with different configuration, Lazy Loading may be disabled (if not, I strongly encourage you to disable it as it is a massive performance bottleneck).

So you have to use the Include Method to eagerly load the related entity.

var instruction = await _context.Instructions.Include(i => i.Property).Where(i => i.Property...).FirstOrDefaultAsync().ConfigureAwait(false);

But before doing this, think if you really need the Instruction . If not, your code could become:

var property = await _context.Properties.Where(p => p.Instructions.Any(i => ...)).FirstOrDefaultAsync().ConfigureAwait(false);

Please note that you have to extend your Property class for this to work to have a back-reference

public partial class Property
{
    // No need for the Key attribute, as this is convention
    public int Id { get; set; }
    public Address Correspondence { get; set; }
    public int CorrespondenceId { get; set; } // Not needed in this scenario, but good practice
    public ICollection<Instruction> Instructions { get; } = new HashSet<Instruction>();
}