title: Invalid foreign key relationship when I try to add a new custom object to an exsting lead
body: <p>I have a Lead and a custom object called Social Account (API Name = Social_Account__c).</p>
  
  <p>I have set a relationship as follows: Lead is a parent of Social Accounts. so a lead has many social accounts.</p>
  
  <p>In the Social Account, I made a custom field named Lead (Data Type: Lookup) to make the relationship.</p>
  
  <p>and here is a lookup detail:</p>
  
  <p>API Name: Lead__c</p>
  
  <p>Related to Lead</p>
  
  <p>Child Relationship Name: Social_Accounts</p>
  
  <p>Related List Label: Social Accounts</p>
  
  <p>I would like to add new social accounts to existing lead if there is a lead with the same email address.</p>
  
  <hr>
  
  <pre><code>Social_Account__c social_account = new Social_Account__c();
  /*add whatever fields on social_account*/
  
  List&lt;Lead&gt; leads =[select Id from Lead where Email =:emailAddress ];
  if(leads.size()&gt;0)
  { 
      Lead existing_lead = new Lead(Id = leads[0].id);
      //ideally i would like to do something like this
      social_account.Lead__c.id = existing_lead.id; //this is where I get an error from
  
      insert social_account;
      update existing_lead;
  }
  </code></pre>
  
  <hr>
  
  <p>but i get a following error message:</p>
  
  <p>Error: Compile Error: Invalid foreign key relationship: Social_Account_<em>c.Lead</em>_c</p>
  
  <p>what am I doing wrong? I would appreciate any suggestions.</p>
  
  <p>thanks</p>