Entity Framework Core Lessons

Getting Id of last inserted Entity

If the entity is using a SQL server identity column as ID, this ID will be populated by EF after a call to SaveChanges.

See this thread on StackOverflow

Relationships

• One to Many TL;DR; use HasOne and WithOne:

// Code in a class that inherits from DbContext

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<ParentEntity>().ToTable(nameof(ParentEntity));
    modelBuilder.Entity<ChildEntity>().ToTable(nameof(ChildEntity));

    modelBuilder
        .Entity<ParentEntity>()
        .HasKey(parent => parent.ParentId);

    modelBuilder
        .Entity<ChildEntity>(builder =>
        {
            builder.HasKey(details => details.MessageDetailId);
            builder
                .HasOne(msg => msg.ParentEntity)
                .WithMany(it => it.ChildEntity)
                .HasForeignKey(it => it.ForeignKeyId);
        });
}

• One to One

• Many to Many https://github.com/aspnet/EntityFramework.Docs/tree/master/samples/core

When queries cannot be evaluated server side

EF Core query could not be translated and will be evaluated locally https://stackoverflow.com/q/45237492/190476

The problem: EF Core could not generate SQL according to the LINQ expression provided. What is the downside: More data would be brought over from database than necessary as the query expression would be evaluated in client (C#) code

• See also, the difference between First, FirstOrDefault, Single and SingleOrDefault

Transactions

When using EF, prefer using DbContext.SaveChanges to manage transactions.

Distributed transactions: https://docs.microsoft.com/en-us/dotnet/api/system.transactions.transactionscope?view=netframework-4.8 NOTE: there is implicit vs explicit transactions. The default is implicit (using TransactionScope) This answer claims distributed transaction isn't supported in cloud scenarios distributed transactions may not scale well either (??) Other considerations:

• why do we need a distributed transaction?