无法使用codefirst在多对多关系中保存连接表中的数据

// 连接表实体类
public class ConnectionEntity
{
    public int Entity1Id { get; set; }
    public int Entity2Id { get; set; }
    public Entity1 Entity1 { get; set; }
    public Entity2 Entity2 { get; set; }
}

// 实体类1
public class Entity1
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<ConnectionEntity> Connections { get; set; }
}

// 实体类2
public class Entity2
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<ConnectionEntity> Connections { get; set; }
}

// 配置多对多关系
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<ConnectionEntity>()
        .HasKey(c => new { c.Entity1Id, c.Entity2Id });

    modelBuilder.Entity<ConnectionEntity>()
        .HasOne(c => c.Entity1)
        .WithMany(e => e.Connections)
        .HasForeignKey(c => c.Entity1Id);

    modelBuilder.Entity<ConnectionEntity>()
        .HasOne(c => c.Entity2)
        .WithMany(e => e.Connections)
        .HasForeignKey(c => c.Entity2Id);
}

// 添加数据
var entity1 = new Entity1 { Name = "Entity1" };
var entity2 = new Entity2 { Name = "Entity2" };

context.Entity1.Add(entity1);
context.Entity2.Add(entity2);
context.SaveChanges();

var connection = new ConnectionEntity
{
    Entity1Id = entity1.Id,
    Entity2Id = entity2.Id
};

context.Connections.Add(connection);
context.SaveChanges();