Elastic search terms facets : problems when performing script_field on an object with array

term_facets.sh

# test performed on elasticsearch-0.19.0.RC2 and elasticsearch-0.19.9

set -x

# delete index
curl -XDELETE '127.0.0.1:9200/test'

# create index
curl -XPUT '127.0.0.1:9200/test'

# mapping
curl -XPUT '127.0.0.1:9200/test/test/_mapping' -d '
{
  "test" : {
    "properties" : {
      "array" : {
        "type" : "object",
        "properties" : {
          "field" : {
            "type" : "string",
            "store" : "yes"
          }
        }
      }
    }
  }
}
'

# index documents
curl -XPOST '127.0.0.1:9200/test/test/1' -d '{
  "array" : [
    { "field" : "a" },
    { "field" : "b" }
  ]
}'
curl -XPOST '127.0.0.1:9200/test/test/2' -d '{
  "array" : [
    { "field" : "b" },
    { "field" : "c" }
  ]
}'

# refresh to make docs available for search
curl -XPOST '127.0.0.1:9200/test/_refresh'

# term facets
curl -XPOST "127.0.0.1:9200/test/test/_search?pretty=true" -d '
{
  "query" : {
    "match_all" : {}
  },
  "facets" : {
    "test" : {
      "terms" : {
        "field" : "array.field",
        "size" : 10
      }
    }
  }
}'
# ...
  #"facets" : {
    #"test" : {
      #"_type" : "terms",
      #"missing" : 0,
      #"total" : 3,
      #"other" : 0,
      #"terms" : [ {
        #"term" : "b",
        #"count" : 2
      #}, {
        #"term" : "c",
        #"count" : 1
      #} ]
    #}
  #}
# OK, but ... where is the "a" value ?
#

# term facets with script on _source
curl -XPOST "127.0.0.1:9200/test/test/_search?pretty=true" -d '
{
  "query" : {
    "match_all" : {}
  },
  "facets" : {
    "test" : {
      "terms" : {
        "script_field" : "_source.array.field",
        "size" : 10
      }
    }
  }
}'
# ...
  #"failures" : [ {
  #"index" : "test",
  #"shard" : 3,
  #"status" : 500,
  #"reason" : "QueryPhaseExecutionException[[test][3]: query[ConstantScore(cache(_type:test))],from[0],size[10]: Query Failed [Failed to execute main query]]; nested: PropertyAccessException[[Error: could not access: field; in class: java.util.ArrayList]\n[Near : {... _source.array.field ....}]\n             ^\n[Line: 1, Column: 1]]; "
#}, {
  #"index" : "test",
  #"shard" : 2,
  #"status" : 500,
  #"reason" : "QueryPhaseExecutionException[[test][2]: query[ConstantScore(cache(_type:test))],from[0],size[10]: Query Failed [Failed to execute main query]]; nested: PropertyAccessException[[Error: could not access: field; in class: java.util.ArrayList]\n[Near : {... _source.array.field ....}]\n             ^\n[Line: 1, Column: 1]]; "
#} ]

# term facets with script on _fields
curl -XPOST "127.0.0.1:9200/test/test/_search?pretty=true" -d '
{
  "query" : {
    "match_all" : {}
  },
  "facets" : {
    "test" : {
      "terms" : {
        "script_field" : "_fields['\''array.field'\'']",
        "size" : 10
      }
    }
  }
}'

# ...
# "facets" : {
    #"test" : {
      #"_type" : "terms",
      #"missing" : 0,
      #"total" : 2,
      #"other" : 0,
      #"terms" : [ {
        #"term" : "org.elasticsearch.search.lookup.FieldLookup@53d34c",
        #"count" : 1
      #}, {
        #"term" : "org.elasticsearch.search.lookup.FieldLookup@1553743",
        #"count" : 1
      #} ]
    #}
  #}
#

I finally found the answer : you have to use the mvel projection
operator to iterate over the collection :
http://mvel.codehaus.org/MVEL+2.0+Projections+and+Folds

I my example, you can do :
{
"terms" : {
"script_field" : "(field in _source.array)",
"size" : 10
}
instead of "script_field" : "_source.array.field"

This does not explains why the "a" element is not taken in account in
the first facets performed on the index, though.