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POHow to solve combined one-to-one and one-to-many relationship in EF 5 Code First
 

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  1. POHow to solve combined one-to-one and one-to-many relationship in EF 5 Code First
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    <p>I´m using Entity Framework 5 and Code First.</p> <p>I have two domain entities <strong>Question</strong> and <strong>Answer</strong> for a quiz application. One question has several possible answers. A question also has one correct answer which should reference one of the possible answers. I am experiencing some issues with the combination of a one-to-many and one-to-one relationship between the to entities. See <strong>Q1</strong> and <strong>Q2</strong>.</p> <p>This is the code for the entities:</p> <pre><code>public class Question { public virtual int Id { get; set; } [Required] public virtual string Text { get; set; } [InverseProperty("Question")] public virtual ICollection&lt;Answer&gt; PossibleAnswers { get; set; } public virtual Answer CorrectAnswer { get; set; } [DatabaseGenerated(DatabaseGeneratedOption.Computed)] public virtual DateTime? UpdateStamp { get; set; } } public class Answer { public virtual int Id { get; set; } [Required] public virtual string Text { get; set; } [ForeignKey("QuestionId")] public virtual Question Question { get; set; } public virtual int QuestionId { get; set; } } </code></pre> <p><strong>Q1:</strong> What should I do to be able to insert the Question object and referenced Answers (thru property PossibleAnswers) in just one roundtrip to the db (eg one call to the contexts SaveChanges)? The error I get when I save the Questions and Answers without adding the Answers first is:</p> <blockquote> <p>Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.</p> </blockquote> <p>To solve that problem I tried the following using fluent API to get the Answers to be added prior to Questions when doing it all with just one call the objectcontexts SaveChanges:</p> <pre><code> protected override void OnModelCreating(DbModelBuilder modelBuilder) { modelBuilder.Entity&lt;Question&gt;() .HasOptional(q =&gt; q.CorrectAnswer) .WithRequired(); base.OnModelCreating(modelBuilder); } </code></pre> <p>However, that lead me to another error:</p> <blockquote> <p>Conflicting changes detected. This may happen when trying to insert multiple entities with the same key.</p> </blockquote> <p>Am I on the right path with the fluent API approach for Q1? Why the error message?</p> <p><strong>Q2</strong>: When deleting a question I realize that there will be an error since the question cannot be deleted before the answers and vice versa. How do I solve this? For instance, is WillCascadeOnDelete supposed to be specified on both Question.CorrectAnswer and Question.PossibleAnswers?</p>
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