这可以用 Entity Framework 4 来完成吗？如果没有，有什么办法可以做到这一点？

Question

我正在制作一个简单的trivial pursuit game。我不确定是否（以及如何）我可以使用 EF4 执行以下操作：-

我的表结构如下。

表格：TrivialPursuitQuestion

=> ID
=> Unique Question
=> AnswerId
=> AnswerType (ie. Geography, Entertainment, etc).

表：GeographyAnswer

=> ID
=> Place Name
=> LatLong

表：娱乐回答：

=> ID
=> Name
=> BordOn
=> DiedOn
=> Nationality .. and other meta data

...等等..

所以当一个人问一个独特的问题时......存储的过程能够弄清楚它是什么类型的答案（即 AnswerType 字段）......并因此查询正确的表。

EG.

SELECT @AnswerId = AnswerId, @AnswerType = AnswerType
FROM TrivialPursuitQuestions
WHERE UniqueQuestion = @Question

IF @AnswerId > 0 AND @AnswerType > 0 BEGIN
    IF @AnswerType = 1
        SELECT * 
        FROM GeographicAnswers 
        WHERE AnswerId = @AnswerID

    IF @AnswerType = 2
        SELECT * 
        FROM EntertainmentAnswer
        WHERE AnswerId = @AnswerId

    ... etc ...
END

现在 .. 我不确定如何使用 EF 执行此操作。首先，存储过程现在可以返回多种结果类型..所以我不确定这是否真的很糟糕。

然后我想，也许存储过程应该返回多个记录集......除了一个包含结果的记录集之外的所有记录集......因为根据设计，只会找到一种答案类型......

EG.

-- Same SELECT as above...


IF @AnswerId > 0 BEGIN
    SELECT * 
    FROM GeographicAnswers 
    WHERE AnswerId = CASE @AnswerId WHEN 1 THEN @AnswerID ELSE 0 END

    SELECT * 
    FROM EntertainmentAnswer 
    WHERE AnswerId = CASE @AnswerId WHEN 2 THEN @AnswerID ELSE 0 END

    ... etc ...
END

这将返回 6 个（多个）记录集......但其中只有一个应该有一些数据。

现在，如果这是一个更好的解决方案......这是否可以使用 EF4 以及如何实现？

我试图避免两次往返数据库，还必须弄清楚要尝试检索的内容......我不想弄清楚......我希望有一些聪明的人对系统进行建模很聪明，可以说“哦！你这是正确的答案'。有点像答案工厂（ala 工厂模式），但使用 Sql Server + EF4。

有人有什么想法吗？

Answer 1

在 EF 设计器中，您可以创建一个名为 Answer 的实体，然后创建从名为 GeographyAnswer、EntertainmentAnswer 的答案派生的实体...在答案实体中使用鉴别器 AnswerType /em>。添加Question 实体。将问题中的关联添加到Answer，将其标记为 1:1。让 EF 为您的数据库生成 DDL。

现在您可以通过question.Answer 获取Answer 以获得Question。您可以查看 Answer 的类型并显示相应的 UI。

不过，这也引出了一个问题，如果它是问题和答案之间的 1:1 对应关系，为什么不使用单个实体 QuestionAnswer 并从中派生出 QuestionAnswerGeography、QuestionAnswerEntertainment、... 现在您可以轻松选择特定类型的Questions：例如dataContext.QuestionAnswers.ofType<QuestionAnswerGeography>()。

这是一个实体图，适用于多个问题可能共享一个答案的情况：

此模型将允许您执行如下查询：

  var geographyQuestions = objectContext.Answers.OfType<Geography>().SelectMany(answer => answer.Questions);

这个模型生成的DDL是：-

-- Creating table 'Answers'
CREATE TABLE [dbo].[Answers] (
    [Id] int IDENTITY(1,1) NOT NULL
);
GO

-- Creating table 'Questions'
CREATE TABLE [dbo].[Questions] (
    [Id] int IDENTITY(1,1) NOT NULL,
    [AnswerId] int  NOT NULL,
    [QuestionText] nvarchar(max)  NOT NULL
);
GO

-- Creating table 'Answers_Geography'
CREATE TABLE [dbo].[Answers_Geography] (
    [PlaceName] nvarchar(max)  NOT NULL,
    [LatLong] nvarchar(max)  NOT NULL,
    [Id] int  NOT NULL
);
GO

-- Creating table 'Answers_Entertainment'
CREATE TABLE [dbo].[Answers_Entertainment] (
    [Name] nvarchar(max)  NOT NULL,
    [BornOn] datetime  NULL,
    [DiedOn] datetime  NULL,
    [Id] int  NOT NULL
);
GO

-- --------------------------------------------------
-- Creating all PRIMARY KEY constraints
-- --------------------------------------------------

-- Creating primary key on [Id] in table 'Answers'
ALTER TABLE [dbo].[Answers]
ADD CONSTRAINT [PK_Answers]
    PRIMARY KEY CLUSTERED ([Id] ASC);
GO

-- Creating primary key on [Id] in table 'Questions'
ALTER TABLE [dbo].[Questions]
ADD CONSTRAINT [PK_Questions]
    PRIMARY KEY CLUSTERED ([Id] ASC);
GO

-- Creating primary key on [Id] in table 'Answers_Geography'
ALTER TABLE [dbo].[Answers_Geography]
ADD CONSTRAINT [PK_Answers_Geography]
    PRIMARY KEY CLUSTERED ([Id] ASC);
GO

-- Creating primary key on [Id] in table 'Answers_Entertainment'
ALTER TABLE [dbo].[Answers_Entertainment]
ADD CONSTRAINT [PK_Answers_Entertainment]
    PRIMARY KEY CLUSTERED ([Id] ASC);
GO

-- --------------------------------------------------
-- Creating all FOREIGN KEY constraints
-- --------------------------------------------------

-- Creating foreign key on [AnswerId] in table 'Questions'
ALTER TABLE [dbo].[Questions]
ADD CONSTRAINT [FK_QuestionAnswer]
    FOREIGN KEY ([AnswerId])
    REFERENCES [dbo].[Answers]
        ([Id])
    ON DELETE NO ACTION ON UPDATE NO ACTION;

-- Creating non-clustered index for FOREIGN KEY 'FK_QuestionAnswer'
CREATE INDEX [IX_FK_QuestionAnswer]
ON [dbo].[Questions]
    ([AnswerId]);
GO

-- Creating foreign key on [Id] in table 'Answers_Geography'
ALTER TABLE [dbo].[Answers_Geography]
ADD CONSTRAINT [FK_Geography_inherits_Answer]
    FOREIGN KEY ([Id])
    REFERENCES [dbo].[Answers]
        ([Id])
    ON DELETE NO ACTION ON UPDATE NO ACTION;
GO

-- Creating foreign key on [Id] in table 'Answers_Entertainment'
ALTER TABLE [dbo].[Answers_Entertainment]
ADD CONSTRAINT [FK_Entertainment_inherits_Answer]
    FOREIGN KEY ([Id])
    REFERENCES [dbo].[Answers]
        ([Id])
    ON DELETE NO ACTION ON UPDATE NO ACTION;
GO