C# 实体框架：删除子实体

我正在使用EF，但我禁用了延迟加载。相反，我使用的是即时加载，因此我创建了自己的服务：

/// <summary>
/// Generic service for entity framework
/// </summary>
/// <typeparam name="T">An entity model</typeparam>
public class Service<T> : IService<T> where T : class
{
    // Create our private properties
    private readonly DbContext _context;
    private readonly DbSet<T> _dbEntitySet;

    /// <summary>
    /// Default constructor
    /// </summary>
    /// <param name="context">The database context</param>
    public Service(DbContext context)
    {
        // Assign our context and entity set
        _context = context ?? throw new ArgumentNullException("context");
        _dbEntitySet = context.Set<T>();
    }

    /// <summary>
    /// Gets all the entities
    /// </summary>
    /// <param name="includes">Option includes for eager loading</param>
    /// <returns></returns>
    public IQueryable<T> List(params string[] includes)
    {

        // Create a query
        IQueryable<T> query = _dbEntitySet;

        // For each include, append to our query
        if (includes != null)
            foreach (var include in includes)
                query = query.Include(include);

        // Return our query
        return query;
    }

    /// <summary>
    /// Creates an entity
    /// </summary>
    /// <param name="model"></param>
    public void Create(T model) => _dbEntitySet.Add(model);

    /// <summary>
    /// Updates an entity
    /// </summary>
    /// <param name="model"></param>
    public void Update(T model) => _context.Entry<T>(model).State = EntityState.Modified;

    /// <summary>
    /// Removes an entity
    /// </summary>
    /// <param name="model"></param>
    public void Remove(T model) => _context.Entry<T>(model).State = EntityState.Deleted;

    /// <summary>
    /// Saves the database context changes
    /// </summary>
    /// <returns></returns>
    public async Task SaveChangesAsync()
    {
        try
        {
            // Save the changes to the database
            await _context.SaveChangesAsync();
        }
        catch (DbEntityValidationException ex)
        {

            // Retrieve the error messages as a list of strings.
            var errorMessages = ex.EntityValidationErrors.SelectMany(x => x.ValidationErrors).Select(x => x.ErrorMessage);

            // Join the list to a single string.
            var fullErrorMessage = string.Join("; ", errorMessages);

            // Combine the original exception message with the new one.
            var exceptionMessage = string.Concat(ex.Message, " The validation errors are: ", fullErrorMessage);

            // Throw a new DbEntityValidationException with the improved exception message.
            throw new DbEntityValidationException(exceptionMessage, ex.EntityValidationErrors);
        }
        catch (DbUpdateException ex)
        {
            throw;
        }
    }

    /// <summary>
    /// Executes a stored procedure in sql
    /// </summary>
    /// <param name="procedure">The name of the sproc</param>
    /// <param name="parameters">the sql params for the sproc</param>
    /// <returns></returns>
    public DbRawSqlQuery<T> ExecuteProcedure(string procedure, List<SqlParameter> parameters)
    {
        var results = _context.Database.SqlQuery<T>($"exec {procedure} { CreateQueryStringFromParams(parameters) }");
        return results;
    }

    /// <summary>
    /// Dispose
    /// </summary>
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    /// <summary>
    /// Creates the input string to run sprocs in sql with EF by converting the sql params into a nice string
    /// </summary>
    /// <param name="parameters"></param>
    /// <returns></returns>
    private static string CreateQueryStringFromParams(IEnumerable<SqlParameter> parameters)
    {
        var response = "";
        var list = parameters as IList<SqlParameter> ?? parameters.ToList();
        var length = list.Count;

        for (var i = 0; i < length; i++)
        {
            response += $"{list[i].ParameterName}=\"{list[i].Value}\"";
            if (i != length - 1)
                response += ", ";
        }

        return response;
    }

    /// <summary>
    /// Disposes of any attached resources
    /// </summary>
    /// <param name="disposing">A boolean indicating whether the object is being disposed</param>
    protected virtual void Dispose(bool disposing)
    {

        // If we are disposing, dispose of our context
        if (disposing)
            _context.Dispose();
    }
}

然后，每个服务将继承此类：

/// <summary>
/// Handles all Group related methods
/// </summary>
public class GroupService : Service<Group>, IGroupService
{

    /// <summary>
    /// The default constructor
    /// </summary>
    /// <param name="unitOfWork"></param>
    public GroupService(DbContext context) : base(context)
    {
    }

    /// <summary>
    /// Lists groups by category
    /// </summary>
    /// <param name="categoryId">The id of the category</param>
    /// <param name="includes"></param>
    /// <returns></returns>
    public IQueryable<Group> List(int categoryId, params string[] includes) => List(includes).Where(m => m.CategoryId == categoryId);

    /// <summary>
    /// Gets a single Group by id
    /// </summary>
    /// <param name="id">The id of the Group</param>
    /// <returns></returns>
    public async Task<Group> GetAsync(int id, params string[] includes) => await List(includes).Where(model => model.Id == id).SingleOrDefaultAsync();
}

每个实体都有一个类似于此GroupService的类。 我还为每种实体类型提供了提供者，以下是我的删除方法：

/// <summary>
/// Delete a Group
/// </summary>
/// <param name="id">The Group id</param>
/// <returns></returns>
public async Task<bool> DeleteAsync(int id)
{

    // Get our model
    var model = await _service.GetAsync(id, "Questions");

    // For each question, remove from the database
    if (model.Questions != null)
        foreach (var question in model.Questions.ToList())
            if (!await _questionProvider.Value.DeleteAsync(question.Id, false))
                throw new Exception("Failed to delete the questions");

    // Update our Questions
    model.Questions = null;

    // Save our model
    _service.Remove(model);

    // Save the database changes
    await _service.SaveChangesAsync();

    // Return true
    return true;
}

如你所见，我只是收回问题

如果有一些问题，那么我调用questionProvider的delete方法，它非常类似：

/// <summary>
/// Delete a question
/// </summary>
/// <param name="id">The question id</param>
/// <param name="saveChanges">Saves the changes to the database after the delete (default true)</param>
/// <returns></returns>
public async Task<bool> DeleteAsync(int id, bool saveChanges = true)
{

    // Get our model
    var model = await _service.GetAsync(id, "Answers");

    // For each answer, delete from the database
    if (model.Answers != null)
        foreach (var answer in model.Answers.ToList())
            if (!await _answerProvider.Value.DeleteAsync(answer.Id, false))
                throw new Exception("Failed to delete the answers");

    // Update our Answers
    model.Answers = null;

    // Save our model
    _service.Remove(model);

    // Save the database changes
    if (saveChanges) await _service.SaveChangesAsync();

    // Return true
    return true;
}

如您所见，在删除所有子项之前，我不会保存上下文更改。现在我必须指出，我并不是直接从父母那里移除孩子。相反，我从它自己的集合中删除实体，然后将属性设置为null。完成所有操作后，我保存更改，但出现以下错误：

操作失败：无法更改关系，因为一个或多个外键属性不可为null。对关系进行更改时，相关外键属性设置为空值。如果外键不支持空值，则必须定义新的关系，必须为外键属性分配另一个非空值，或者必须删除不相关的对象

---

有人知道我如何用类似于我试图实现的方法删除实体吗？

在阅读了其他一些有类似问题的人的文章后，这很容易解决。我刚将Service.cs中的一行代码从：

这起作用了

更新

我还发现，渴望加载需要包括所有将受到影响的导航属性。 因此，如果我要删除一个有答案和答案有公式的问题，Get调用必须是：

var model = await _service.GetAsync(id, "Answers.Formulas");

如果你不包括这一点，你会得到一个错误

public void Remove(T model) => _dbEntitySet.Remove(model);

var model = await _service.GetAsync(id, "Answers.Formulas");