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    <p>I am not sure if you want the following:</p> <pre class="lang-cs prettyprint-override"><code>modelBuilder.Entity&lt;Customers&gt;() .HasMany(c =&gt; c.Phones) .WithOptional() .Map(m =&gt; m.MapKey("CustomerPhoneId")); modelBuilder.Entity&lt;Companies&gt;() .HasMany(c =&gt; c.Phones) .WithOptional() .Map(m =&gt; m.MapKey("CompanyPhoneId")); </code></pre> <p>Using <code>MapKey</code> is optional. It just gives the foreign key column the name you want. If you omit this EF will create a standard name (something with underscore: <code>..._Id</code>).</p> <p>In fact the whole mapping is optional because the mapping conventions will just create the same relationships without any Fluent API mapping at all.</p> <p>The <code>Phones</code> table will have two nullable foreign keys <code>CustomerPhoneId</code> and <code>CompanyPhoneId</code> refering to <code>Customers</code> table and <code>Companies</code> table respectively.</p> <p><strong>Edit</strong></p> <p>An alternative which would only require one foreign key in the <code>Phone</code> table for multiple different entities would be an inheritance mapping:</p> <pre class="lang-cs prettyprint-override"><code>public abstract class OrganizationsWithPhone { public OrganizationsWithPhone() { Phones = new List&lt;Phones&gt;(); } public Guid Id { get; set; } public ICollection&lt;Phones&gt; Phones { get; set; } } public class Customers : OrganizationsWithPhone { public string FirstName { get; set; } public string LastName { get; set; } } public class Companies : OrganizationsWithPhone { public string Name { get; set; } } public class Phones { public Guid Id { get; set; } public string Number { get; set; } public string Extension { get; set; } } </code></pre> <p>Mapping:</p> <pre class="lang-cs prettyprint-override"><code>modelBuilder.Entity&lt;OrganizationsWithPhone&gt;() .HasMany(o =&gt; o.Phones) .WithOptional() // or .WithRequired() .Map(m =&gt; m.MapKey("OrganizationsWithPhoneId")); modelBuilder.Entity&lt;OrganizationsWithPhone&gt;() .ToTable("OrganizationsWithPhone"); modelBuilder.Entity&lt;Customers&gt;() .ToTable("Customers"); modelBuilder.Entity&lt;Companies&gt;() .ToTable("Companies"); </code></pre> <p>Now you have only one foreign key relationship between <code>Phones</code> and the base table <code>OrganizationsWithPhone</code>, but due to the inheritance mapping there are additional one-to-one relationships between the base table and the tables for the derived entities <code>Customers</code> and <code>Companies</code>. Basically the number of necessary relationships stays the same (or is even one more in this model).</p> <p>In this model a customer and a company cannot share the same phone numbers because a row in the <code>Phones</code> table refers to an <code>OrganizationsWithPhone</code> which can either be a customer or a company but not both at the same time.</p> <p>The base table <code>OrganizationsWithPhone</code> has only a single column <code>Id</code>. If you have more common properties in all derived entities you can move them into the base entitiy.</p>
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    1. COI don't like the approach cause if I have any other entity that will need to have phones it will required another column.
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    2. CO@Son_of_Sam: See my Edit, there is an alternative solution.
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