Question

I have a Dbcontext where in its OnModelCreating() method I have something like this:

     modelBuilder.Entity<CmsContentData>().HasMany<CmsKeyword>(m => 
    m.CmsKeywords).WithMany(m => m.CmsContentDatas).Map(m =>
    {
    m.ToTable("CmsContentData_CmsKeywords");
    m.MapLeftKey("CmsContentDataID");
    m.MapRightKey("CmsKeywordID");
    });

I want to read from the CmsContentData_CmsKeywords table but I wonder how ? (there is no CmsContentData_CmsKeywords model in the project solution)

Answer 1

显然，您已经设计了一个多对多关系：每个CmsContentData具有零个或多个CmsKeyWords，每个CmsKeyword被零个或多个CmsContentData使用。 / p>

在关系数据库中，此多对多关系是使用联结表实现的。该表是您在DbContext.OnModelCreating中提到的表。

是的，您不会将此联结表添加为DbContext中的DbSet。您的课程如下：

class CmsContentData
{
    public int Id {get; set;}

    // every CmsContentData has zero or more CmsKeyWords (many-to-many)
    virtual ICollection<CmsKeyWord> CmsKeyWords {get; set;}

    ... // other properties
}
class CmsKeyWord
{
    public int Id {get; set;}

    // every CmsKeyWord is used by zero or more CmsContentData (many-to-many)
    virtual ICollection<CmsContentData> CmsContentData{get; set;}

    ... // other properties
}
class MyDbContext : Dbcontext
{
    public DbSet<CmsContentData> CmsContentData {get; set;}
    public DbSet<CmsKeyWord> CmsKeyWords {get; set;}
}

这是Entity Framework检测到您设计了多对多关系所需要知道的一切。即使没有在OnModelCreating中的代码，它也会为您和联结表创建两个表。

仅当您对表和列的默认标识符不满意时，才需要在OnModelCreating中使用代码。

但是，如果我没有引用联结表，该如何进行联接？

答案：不要（组）加入，请使用ICollections

示例：获取带有其所有（或部分）其CmsKeyWords的CmsContentData：

var result = dbContext.CmsContextData
    .Where(cmsContextData => ...)        // only take certain cmsContextData
    .Select(cmsContextData => new        // only select properties you plan to use:
    {
        Id = cmsContextData.Id,
        Name = cmsContextData.Name,
        ...

        Keywords = cmsContextData.CmsKeywords
            .Where(keyWord => keyWord.StartsWith(...))   // only select certain keywords
            .Select(keyWord => new                       // only select properties you plan to use
            {
                 Id = keyword.Id,
                 Text = keyWord.Text,
                 ...
            })
            .ToList(),
    });

实体框架足够聪明，可以检测到（组）与您的两个表联接，并且需要联接表。

反过来：

选择所有（或某些）使用某些特定CmsKeyWords的CmsContentData：

var result = dbContext.CmsKeyWords
    .Where(keyWord => keyWord.StartsWith(...)  // take only certain keywords
    .Select(keyword => new                     // select only the properties you plan to use
    {
         Text = keyWord.Text,
         ...

         CmsContextData = keyWord.CmsContextData  // I only want specific context data
             .Where(contextData => ...)           // that use this Keyword
             .select(contextData => new
             {
                 ... // ContextData properties you plan to use
             });
    });

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