有点盲目，因为你没有包括什么属性实体有。但您应该有一个关系的属性为 CustomerGroups 。只需将该属性设置为要与之相关的组。例如，这将创建一个新的组名称foo bar，并将实体与该组关联。

Flying a little blind since you didn't include what properties entity has. But you should have a property for the relationship to CustomerGroups. Just set that property with the groups you want to be related to. For example, this would create a new Group name "foo bar" and relate the entity to that group.

using (var context = DataObjectFactory.CreateContext())
{
    entity.CustomerGroups = new List<CustomerGroup> { GroupName = "Foo bar" };
    context.Customers.Add(entity);
    context.SaveChanges();
    return entity.Id;
}

如果关系设置正确，EF会自动将记录插入 CustomerGroups 并在 CustomerInGroups 表中插入一个关系。

If the relationship is setup correctly, EF will automatically insert a record into CustomerGroups and insert a relationship into the CustomerInGroups table.

编辑：

如果您尝试向现有的客户添加一个现有的 CustomerGroup 您首先需要从数据库中获取 CustomerGroup ，然后将其添加到您要插入的Customer实体。

If you're trying to add an existing CustomerGroup to a new Customer. You'll want to get the CustomerGroup from the database first, then add it to the Customer entity you're inserting.

using (var context = DataObjectFactory.CreateContext())
{
    var customerGroups = context.CustomerGroups.Where(...).ToList(); // get your CustomerGroup object(s) here, and ensure it's enumerated with ToList()
    entity.CustomerGroups = customerGroups;
    context.Customers.Add(entity);
    context.SaveChanges();
    return entity.Id;
}