Field Alias error when used in collapse query

[pcsanwald]

Elasticsearch version : 7.x

Plugins installed: none
JVM version: java version "10" 2018-03-20
OS version: 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64

Description of the problem including expected versus actual behavior:

Collapsing on a field alias returns an NPE, rather than the expected result of the collapse as applied to the field pointed to via the alias' path.

Steps to reproduce:

0. Bring up a local instance of elasticsearch, running latest master.

1. Index some docs using python script attached (you may need to pip install elasticsearch, pip install faker for the script to work. Note that postal_code is a field alias, pointing to zip_code, a long. populate.py.gz

2. Run the following query

curl "http://localhost:9200/twitter/_search/" \
     -H 'Content-Type: application/json' \
     -d $'{
    "query": {
        "match_all": {}
    },
    "collapse" : {
        "field" : "postal_code",
        "inner_hits": [
            {
              "name": "by_postal_code",
                "size": 3,
                "collapse" : {"field" : "name"},
                "sort": ["likes"]
            }
        ]
    }
  }' | jq '.'

Observe the error.

3. Contrast this response with the same query, but using the zip_code field instead

curl "http://localhost:9200/twitter/_search/" \
     -H 'Content-Type: application/json' \
     -d $'{
    "query": {
        "match_all": {}
    },
    "collapse" : {
        "field" : "zip_code",
        "inner_hits": [
            {
              "name": "by_postal_code",
                "size": 3,
                "collapse" : {"field" : "name"},
                "sort": ["likes"]
            }
        ]
    }
  }' | jq '.'

This query returns results as expected.
Provide logs (if relevant): The stacktrace for the error in step 2 is below.

[elasticsearch] [2018-08-03T13:28:42,185][DEBUG][o.e.a.s.TransportSearchAction] [node-0] Failed to execute [SearchRequest{searchType=QUERY_THEN_FETCH, indices=[twitter], indicesOptions=IndicesOptions[ignore_unavailable=false, allow_no_indices=true, expand_wildcards_open=true, expand_wildcards_closed=false, allow_aliases_to_multiple_indices=true, forbid_closed_indices=true, ignore_aliases=false], types=[], routing='null', preference='null', requestCache=null, scroll=null, maxConcurrentShardRequests=0, batchedReduceSize=512, preFilterShardSize=128, allowPartialSearchResults=true, source={"query":{"match_all":{"boost":1.0}},"collapse":{"field":"postal_code","inner_hits":{"name":"by_postal_code","ignore_unmapped":false,"from":0,"size":3,"version":false,"explain":false,"track_scores":false,"sort":[{"likes":{"order":"asc"}}],"collapse":{"field":"name"}}}}}] while moving to [expand] phase
[elasticsearch] java.lang.NullPointerException: null
[elasticsearch]         at org.elasticsearch.action.search.ExpandSearchPhase.run(ExpandSearchPhase.java:79) ~[elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.AbstractSearchAsyncAction.executePhase(AbstractSearchAsyncAction.java:160) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.AbstractSearchAsyncAction.executeNextPhase(AbstractSearchAsyncAction.java:153) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.FetchSearchPhase.moveToNextPhase(FetchSearchPhase.java:205) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.FetchSearchPhase.lambda$innerRun$2(FetchSearchPhase.java:104) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.FetchSearchPhase.innerRun(FetchSearchPhase.java:110) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.FetchSearchPhase.access$000(FetchSearchPhase.java:44) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.action.search.FetchSearchPhase$1.doRun(FetchSearchPhase.java:86) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:723) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.TimedRunnable.doRun(TimedRunnable.java:41) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-7.0.0-alpha1-SNAPSHOT.jar:7.0.0-alpha1-SNAPSHOT]
[elasticsearch]         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135) [?:?]
[elasticsearch]         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
[elasticsearch]         at java.lang.Thread.run(Thread.java:844) [?:?]

[pcsanwald]

Discussed with @jtibshirani before opening to confirm that this seems like an actual bug; we're in agreement that it does seem like an actual issue. I did not test this on 6.x but suspect the same behavior will hold.

[elasticmachine]

Pinging @elastic/es-search-aggs