title: LINQ to SQL : Why can't I have a Zero to Many Cardinality
body: <p>I created a foreign key relationship in my database so that I could access another table as a property in this case <strong>House.Area</strong></p>
  
  <p>Now if I create a <strong>House</strong> object and have <strong>Area</strong> as null I get the following exception on <code>SubmitChanges()</code>:</p>
  
  <blockquote>
    <p>An attempt was made to remove a relationship between a Area and a House. However, one of the relationship's foreign keys (House.AreaID) cannot be set to null.</p>
  </blockquote>
  
  <p>Ok i made the example above a little simpler, but a comment below make me think i should give a better example</p>
  
  <p>"House" Table has a column called CityID  mapping to a "City" Table  connecting on CityID as FK and CityID can't be null</p>
  
  <p>"House" Table also has a column called AreaID mapping to a "Area" Table connecting on CityID as well as AreaID, but AreaID can be null.</p>
  
  <p>House needs to have 1 city always. 1 city can have many houses.
  House can be in zero or 1 area. 1 area might have zero or many houses.</p>
  
  <p>I update the 
  House.City = new City(....);
  House.CityID does get a value &lt;- checked</p>
  
  <p>Error Update</p>
  
  <blockquote>
    <p>An attempt was made to remove a relationship between a Area and a House. However, one of the relationship's foreign keys (House.CityID, House.AreaID) cannot be set to null.</p>
  </blockquote>
  
  <p>Unless updating the Area is overriding the value of CityID which can explain the error. Comments please. Any way around?</p>