title: Neo4J : Force relationship constrains
body: <p>I have two type of objects say
       Entity and Type.</p>
  
  <p>Each entity can be of of one Type only.</p>
  
  <p>So </p>
  
  <pre><code> Merge (M1:Entity {name:'m1'})
   Merge (M2:Entity {name:'m2'})
   Merge (T1:EType {name:'System'})
   Merge (T2:EType {name:'User'})
  </code></pre>
  
  <p>Then </p>
  
  <pre><code>MERGE (M1)-[:typeOf]-&gt;(T1) 
  MERGE (M2)-[:typeOf]-&gt;(T1)
  </code></pre>
  
  <p>should work, but following one should give me error ( Each entity should be linked with only one EType)</p>
  
  <pre><code>MERGE (M1)-[:typeOf]-&gt;(T2)
  </code></pre>
  
  <p>What I need is to provide constrains on the schema level.</p>