Applying a filter to the last x nested documents in the selected period, to find if one of them match a condition

Hi,

I'm trying to retrieve the number of users who bought the product "123" at least once in their last three sales in the selected period.

With this data structure :

mapping :

{
  "properties": {
    "user_id": {
      "type": "keyword"
    },
    "sales": {
      "type": "nested",
      "properties": {
        "sale_id": {
          "type": "keyword"
        },
        "date": {
          "type": "date"
        },
        "sales": {
          "type": "nested",
          "properties": {
            "product_id": {
              "type": "keyword"
            },
            "quantity": {
              "type": "integer"
            }
          }
        }
      }
    }
  }
}

data example :

{
  "user_id": "1",
  "sales": [
    {
      "sale_id": "1",
      "date": "2021-03-01",
      "products": [
        {
          "product_id": "123",
          "quantity": 1
        },
        {
          "product_id": "456",
          "quantity": 2
        }
        ...
      ]
    },
    {
      "sale_id": "2",
      "date": "2020-02-01",
      "products": [
        {
          "product_id": "123",
          "quantity": 4
        },
        {
          "product_id": "789",
          "quantity": 3
        }
        ...
      ]
    }
  ]
}

I wrote a query that assigns a score of 1000000 per sale containing the product I'm looking for to filtre user by min_score, but I can't limit the scoring to the last 3 sales.

I tried to add an inner_hits in the nested filter but it is not taken into account in the score calculation.

This is my current query :

{
  "size": 0,
  "track_total_hits": true,
  "query": {
    "bool": {
      "must": [
        {
          "function_score": {
            "min_score": 1000000,
            "score_mode": "sum",
            "query": {
              "bool": {
                "must": [
                  {
                    "nested": {
                      "inner_hits": {
                        "size": 3,
                        "sort": {
                          "sales.date": "DESC"
                        }
                      },
                      "path": "sales",
                      "score_mode": "sum",
                      "query": {
                        "function_score": {
                          "min_score": 1,
                          "query": {
                            "bool": {
                              "must": [
                                {
                                  "constant_score": {
                                    "filter": {
                                      "range": {
                                        "sales.date": {
                                          "gte": "2019-01-01",
                                          "lte": "2020-01-01"
                                        }
                                      }
                                    },
                                    "boost": 1
                                  }
                                }
                              ],
                              "should": [
                                {
                                  "constant_score": {
                                    "filter": {
                                      "terms": {
                                        "sales.products.product_id": [
                                          "123"
                                        ]
                                      }
                                    },
                                    "boost": 1000000
                                  }
                                }
                              ]
                            }
                          }
                        }
                      }
                    }
                  }
                ]
              }
            }
          }
        }
      ]
    }
  }
}

Is it possible to isolate the last 3 sales of each user according to the applied date filter, then to apply on these 3 sales a filter to know if at least one of them contains the searched product?

Thanks for your help !