Query the latest document of each type on Elasticsearch

First, please note that I had to change your mapping for the headline field to string, as in your sample documents headlines are strings and not objects.

So, a query like the following one would retrieve what you expect:

curl -XPOST "$ELASTICSEARCH_ENDPOINT/news/_search" -d '{  "size": 0,  "query": {    "filtered": {      "filter": {        "term": {          "user": "John"           <--- filter for user=John        }      }    }  },  "aggs": {    "sources": {      "terms": {        "field": "source"          <--- aggregate by source      },      "aggs": {        "latest": {          "top_hits": {            "size": 1,             <--- only take the first...            "_source": [           <--- only the date and headline               "headline",               "timestamp"            ],            "sort": {              "timestamp": "desc"  <--- ...and only the latest hit            }          }        }      }    }  }}'

That will yield something like this:

{  ...  "aggregations" : {    "sources" : {      "doc_count_error_upper_bound" : 0,      "sum_other_doc_count" : 0,      "buckets" : [ {        "key" : "CNN",        "doc_count" : 2,        "latest" : {          "hits" : {            "total" : 2,            "max_score" : null,            "hits" : [ {              "_index" : "news",              "_type" : "news",              "_id" : "AU7Sh3VDGDddn2ZNuDVl",              "_score" : null,              "_source":{                  "headline": "More great news",                   "timestamp": "2015-07-28T00:08:23.000"              },              "sort" : [ 1438042103000 ]            } ]          }        }      }, {        "key" : "ESPN",        "doc_count" : 2,        "latest" : {          "hits" : {            "total" : 2,            "max_score" : null,            "hits" : [ {              "_index" : "news",              "_type" : "news",              "_id" : "AU7Sh3VDGDddn2ZNuDVn",              "_score" : null,              "_source":{                   "headline": "More sports news",                    "timestamp": "2015-07-28T00:10:35.000"              },              "sort" : [ 1438042235000 ]            } ]          }        }      } ]    }  }}