Elasticsearch English stemming not working correctly

I've added an english stemmer analyzer and filter to our query but it doesn't seem to be working correctly with plurals stemming from 'y' => 'ies'. For example, when I search 'raspberry' the results never include 'raspberries' and so on. I've tried both english and minimal_english but I still get the same result.

Here's the analyzer and settings:

   analysis: {
     analyzer: {
       custom_analyzer: {
         type: "custom",
         tokenizer: "standard",
         filter: ["lowercase", "english_stemmer"],
       },
     },
     filter: {
       english_stemmer: {
         type: "stemmer",
         language: "english",
       },
     },
   },
 }

What am I doing wrong?

Though english should work for the e.g. you mentioned, you can even go for porter_stem instead. This is equivalent to stemmer with language english.

porter_stem in action:

POST /_analyze
{
  "tokenizer": "standard",
  "filter": ["porter_stem"],
  "text": ["raspberry", "raspberries"]
}

Response of above request:

{
  "tokens" : [
    {
      "token" : "raspberri",
      "start_offset" : 0,
      "end_offset" : 9,
      "type" : "<ALPHANUM>",
      "position" : 0
    },
    {
      "token" : "raspberri",
      "start_offset" : 10,
      "end_offset" : 21,
      "type" : "<ALPHANUM>",
      "position" : 101
    }
  ]
}

You can see both raspberry and raspberries get tokenise to raspberri. Therefore searching for raspberry will also match raspberries and vice-versa.

Make sure that the field against which you are indexing and searching has defined the analyzer as custom_analyzer (according to settings you stated in your question).

Working e.g.

Mapping:

PUT test
{
  "settings": {
    "analysis": {
      "analyzer": {
        "custom_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "lowercase",
            "english_stemmer"
          ]
        }
      },
      "filter": {
        "english_stemmer": {
          "type": "stemmer",
          "language": "english"
        }
      }
    }
  },
  "mappings": {
    "properties": {
      "field1": {
        "type": "text",
        "analyzer": "custom_analyzer"
      }
    }
  }
}

Indexing:

PUT test/_doc/1
{
  "field1": "raspberries"
}

PUT test/_doc/2
{
  "field1": "raspberry"
}

Search:

GET test/_search
{
  "query": {
    "match": {
      "field1": {
        "query": "raspberry"
      }
    }
  }
}

Response:

{
  "took" : 0,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 2,
      "relation" : "eq"
    },
    "max_score" : 0.18232156,
    "hits" : [
      {
        "_index" : "test",
        "_type" : "_doc",
        "_id" : "1",
        "_score" : 0.18232156,
        "_source" : {
          "field1" : "raspberries"
        }
      },
      {
        "_index" : "test",
        "_type" : "_doc",
        "_id" : "2",
        "_score" : 0.18232156,
        "_source" : {
          "field1" : "raspberry"
        }
      }
    ]
  }
}

You can also have a look at other stemmer kstem.