Elastic Search search query not returning when I search for synonyms

So I am trying to implement synonyms (at search time) in Elastic Search, so far i've defined synonyms like this in Kibana:

PUT /<my_index>/_settings
{
 "settings": {
    "index": {
      "analysis": {
        "filter": {
          "synonym": {
            "analyzer": "standard",
            "search_analyzer": "synonym_analyzer",
            "type": "synonym",
            "synonyms": [
              "foo => bar"
            ]
          }
        }
      }
    }
  }
}

But then when I try to search for foo I dont get back anything.

My search request looks like this:

GET <my_index>/_search
{
  "size": "12",
  "from": "0",
  "min_score": "0.1",
  "query": {
    "function_score": {
      "functions": [
        {
          "field_value_factor": {
            "field": "product.sales_count",
            "missing": 0,
            "modifier": "log1p"
          }
        },
        {
          "field_value_factor": {
            "field": "product.image_count",
            "missing": 0,
            "modifier": "log1p"
          }
        }
      ],
      "query": {
        "bool": {
          "filter": [
            {
              "term": {
                "product.is_visible": true
              }
            }
          ],
          "should": [
            {
              "query_string": {
                "default_field": "product.i18ns.ro_RO.*.analyzed",
                "query": "foo",
                "analyzer": "synonym_analyzer"
              }
            },
            {
              "query_string": {
                "default_field": "categories.i18ns.ro_RO.*.analyzed",
                "query": "foo",
                "analyzer": "synonym_analyzer"
              }
            },
            {
              "query_string": {
                "default_field": "brands.i18ns.ro_RO.*.analyzed",
                "query": "foo",
                "analyzer": "synonym_analyzer"
              }
            },
            {
              "query_string": {
                "default_field": "product.images.i18ns.ro_RO.*.analyzed",
                "query": "foo",
                "analyzer": "synonym_analyzer"
              }
            },
            {
              "query_string": {
                "default_field": "features.i18ns.ro_RO.*.analyzed",
                "query": "foo",
                "analyzer": "synonym_analyzer"
              }
            },
            {
              "query_string": {
                "default_field": "product.ref",
                "query": "foo",
                "boost": 2     
              }
            }
          ]
        }
      }
    }
  },
  "aggs": {
    "by_categories": {
      "terms": {
        "field": "categories.i18ns.ro_RO.title.raw",
        "size": 100
      }
    }
  }
}

NOTE: I do have bar in my index.

There is some issue with the way you have defined the synonym analyzer. The analyzer part must come under the analysis part and not under the filter part of the index setting. Refer to this official documentation, to know more about the settings of synonym token filter

Adding a working example with index data, mapping, search query, and search result.

Index Mapping:

{
  "settings": {
    "index": {
      "analysis": {
        "filter": {
          "synonym_filter": {
            "type": "synonym",
            "synonyms": [
              "foo => bar"
            ]
          }
        },
        "analyzer": {
          "synonym_analyzer": {
            "filter": [
              "synonym_filter"
            ],
            "tokenizer": "standard"
          }
        }
      }
    }
  },
  "mappings": {
    "properties": {
      "title": {
        "type": "text"
      }
    }
  }
}

Index Data:

{
  "title": "bar"
}

Search Query:

{
  "query": {
    "match": {
      "title": {
        "query": "foo",
        "analyzer": "synonym_analyzer"
      }
    }
  }
}

Search Result:

"hits": [
      {
        "_index": "67566386",
        "_type": "_doc",
        "_id": "2",
        "_score": 0.9808292,
        "_source": {
          "title": "bar"
        }
      }
    ]