ES 5.4 - Field Collapsing + inner hits throws exception

collapse.inner_hits.bug.txt

## Example gist, showing a possible bug in inner_hits workings in collapse feature when size > amount of results.

DELETE webshop

#Create some products
POST webshop/product/1
{
  "title": "The Division - PS4",
  "family_id": 1
}

POST webshop/product/2
{
  "title": "The Division - XBOX One",
  "family_id": 1
}

POST webshop/product/3
{
  "title": "World of Warcraft",
  "family_id": 2
}

#This query works as expected, just a simple collapse
GET webshop/_search
{
  "from": 0, 
  "size": 20, 
  "query": {
    "match": {
      "title": "The Division"
    }
  },
  "collapse": {
    "field": "family_id"
  }
}

#This query works as expected, just a simple collapse with inner_hits, requesting more products than are available. Empty results responded
GET webshop/_search
{
  "from": 5, 
  "size": 20, 
  "query": {
    "match": {
      "title": "The Division"
    }
  },
  "collapse": {
    "field": "family_id"
  }
}

#This query works as expected, just a simple collapse including inner_hits
GET webshop/_search
{
  "from": 0, 
  "size": 20, 
  "query": {
    "match": {
      "title": "The Division"
    }
  },
  "collapse": {
    "field": "family_id",
    "inner_hits" : {
      "name" : "rollup_hits"
    }
  }
}

#FAILS - The same collapse as before, with inner_hits but now size in query is more than amount of documents returned by query.
GET webshop/_search
{
  "from": 5, 
  "size": 20, 
  "query": {
    "match": {
      "title": "The Division"
    }
  },
  "collapse": {
    "field": "family_id",
    "inner_hits" : {
      "name" : "rollup_hits"
    }
  }
}

#RESULT:
{
  "error": {
    "root_cause": [
      {
        "type": "remote_transport_exception",
        "reason": "[Byron][127.0.0.1:9300][indices:data/read/msearch]"
      }
    ],
    "type": "action_request_validation_exception",
    "reason": "Validation Failed: 1: no requests added;"
  },
  "status": 400
}