Elasticsearch distinct filter values

I have a large document store in elasticsearch and would like to retrieve the distinct filter values for display on HTML drop-downs.

An example would be something like

[
    {
        "name": "John Doe",
        "deparments": [
            {
                "name": "Accounts"
            },
            {
                "name": "Management"
            }
        ]
    },
    {
        "name": "Jane Smith",
        "deparments": [
            {
                "name": "IT"
            },
            {
                "name": "Management"
            }
        ]
    }
]

The drop-down should have a list of departments, i.e. IT, Account and Management.

Would some kind person please point me in the right direction for retrieving a distinct list of departments from elasticsearch?

Thanks

This is a job for a terms aggregation (documentation).

You can have the distinct departments values like this :

POST company/employee/_search
{
  "size":0,
  "aggs": {
    "by_departments": {
      "terms": {
        "field": "departments.name",
        "size": 0 //see note 1
      }
    }
  }
}

Which, in your example, outputs :

{
   ...
   "aggregations": {
      "by_departments": {
         "buckets": [
            {
               "key": "management", //see note 2
               "doc_count": 2
            },
            {
               "key": "accounts",
               "doc_count": 1
            },
            {
               "key": "it",
               "doc_count": 1
            }
         ]
      }
   }
}

Two additional notes :

setting size to 0 will set the maximum buckets number to Integer.MAX_VALUE. Don't use it if there are too many departments distinct values.
you can see that the keys are terms resulting of analyzing departments values. Be sure to use your terms aggregation on a field mapped as not_analyzed .

For example, with our default mapping (departments.name is an analyzed string), adding this employee:

{
  "name": "Bill Gates",
  "departments": [
    {
      "name": "IT"
    },
    {
      "name": "Human Resource"
    }
  ]
}

will cause this kind of result:

{
   ...
   "aggregations": {
      "by_departments": {
         "buckets": [
            {
               "key": "it",
               "doc_count": 2
            },
            {
               "key": "management",
               "doc_count": 2
            },
            {
               "key": "accounts",
               "doc_count": 1
            },
            {
               "key": "human",
               "doc_count": 1
            },
            {
               "key": "resource",
               "doc_count": 1
            }
         ]
      }
   }
}

With a correct mapping :

POST company
{
  "mappings": {
    "employee": {
      "properties": {
        "name": {
          "type": "string"
        },
        "departments": {
          "type": "object",
          "properties": {
            "name": {
              "type": "string",
              "index": "not_analyzed"
            }
          }
        }
      }
    }
  }
}

The same request ends up outputting :

{
   ...
   "aggregations": {
      "by_departments": {
         "buckets": [
            {
               "key": "IT",
               "doc_count": 2
            },
            {
               "key": "Management",
               "doc_count": 2
            },
            {
               "key": "Accounts",
               "doc_count": 1
            },
            {
               "key": "Human Resource",
               "doc_count": 1
            }
         ]
      }
   }
}

Hope this helps!