title: Unique Constraint in Elasticsearch
body: <p>I need to reject at indexing time if same value in the same field is found in a different document.For example if I register with one username ,next time I should not register with the same username, like how we are using unique constraint in other databases oracle,mysql etc...Can someone help me how to achieve this in ElasticSearch..</p>
  
  <p>Thanks,
  SP</p>