如何仅使用Include预加载ICollection的子级属性？

解决方案 #

1. EF Core：使用ThenInclude实现链式预加载 #

原有仓储的参数类型Expression<Func<Loan, object>>[]无法支持嵌套导航属性的预加载逻辑，需将参数改为Func<IQueryable<Loan>, IQueryable<Loan>>[]，以此传入包含Include和ThenInclude的完整查询构建逻辑：

修改后的仓储层代码

public Loan LoadByLoanNumberWithRequiredData(string loanNumber, params Func<IQueryable<Loan>, IQueryable<Loan>>[] includes)
{
    var query = Context.Loans.AsQueryable();
    foreach (var include in includes)
    {
        query = include(query);
    }
    return query.FirstOrDefault(l => l.LoanNumber == loanNumber);
}

业务层调用示例

Loan loan = LoanUnitOfWork.LoanRepository.LoadByLoanNumberWithRequiredData(
    loanNumber,
    q => q.Include(l => l.LoanStatuses).ThenInclude(ls => ls.Status)
);

此方式可一次性预加载LoanStatuses及其关联的Status属性，彻底避免N+1查询问题。

2. EF6及更早版本：使用字符串路径重载Include #

EF6及更早版本不支持ThenInclude，可通过指定字符串形式的嵌套导航路径实现预加载：

业务层调用示例

Loan loan = LoanUnitOfWork.LoanRepository.LoadByLoanNumberWithRequiredData(
    loanNumber,
    l => l.LoanStatuses,
    "LoanStatuses.Status"
);

注意：字符串路径无编译时校验，需确保拼写正确。

3. 兼容方案：使用Select指定嵌套属性 #

若不想修改仓储方法的参数类型，可通过Select表达式指定集合导航属性的子属性：

业务层调用示例

Loan loan = LoanUnitOfWork.LoanRepository.LoadByLoanNumberWithRequiredData(
    loanNumber,
    l => l.LoanStatuses.Select(ls => ls.Status)
);

此方式在EF6及EF Core中均支持，但仅适用于嵌套层级较少的场景。

内容的提问来源于stack exchange，提问作者sebastian whiffen