IT:AD:EF/CodeFirst:HowTo:Define Models/Relationships/Examples/n-1 [Notes]

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# IT:AD:EF/CodeFirst:HowTo:Define Models/Relationships/Examples/n-1 #



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<panel title="Summary">

An invoice has to be categorized, so the category is mandatory. And one category can be shared between many invoices, so from *its point of view*, it's 1-* (`HasMany()`).

<uml>
invoice "*" --> "1" category : references
</uml>


</panel>


## Process ##

<sxh csharp>
    //Many to One, Required
            ////0...*-1:
            modelBuilder.Entity<Invoice>()
                //Ensures Invoice has a Category applied before it is saved:
                .HasRequired(i => i.Category)
                //Ensure (1) Category has many (*) Invoices
                .WithMany(); //1-n
</sxh>


Common is to use n-1 for mandatory reference data, and use an FK property to set it:

<WRAP tip>
Working with an FK is intelligent as it doesn't force you to have to fetch the InvoiceType first, in order to set the Type property -- just set the TypeFK to a [[IT/About/#WellKnown/]] value:
</WRAP>

<sxh csharp>
    modelBuilder.Entity<Invoice>()
                .HasRequired(m => m.InvoiceType)
                .WithMany()
                .HasForeignKey(m => m.InvoiceTypeFK);
</sxh>