Json.Decode：提取列表项并将其展平/合并到记录中

Question

我想将一些JSON（带有inner_hits的Elasticsearch搜索结果）解析为一个平面记录，但是记录的某些字段应该来自列表中的一个项目，该列表又位于一个嵌套对象中：

{
    "_index": "test",
    "_type": "doc",
    "_id": "AUG.02.013.1320.02630.0",
    "_score": null,
    "_routing": "1",
    "_source": {
        "child_mgrpid": "1.1",
        "child_varia": "blabla",
        "type": "child",
        "my_join_field": {
            "name": "child",
            "parent": "AUG.02.013.1320"
        },
        "@version": "1",
        "@timestamp": "2020-01-12T16:45:11.302Z",
    },
    "inner_hits": {
        "prnt": {
            "hits": {
                "total": 1,
                "max_score": null,
                "hits": [
                    {
                        "_index": "test",
                        "_type": "doc",
                        "_id": "AUG.02.013.1320",
                        "_score": null,
                        "_routing": "1",
                        "_source": {
                            "pt_archiv": "",
                            "pt_id": "AUG.02.013.1320",
                            "pt_titel": "",
                            "pt_l_id": "AUG.02.013",
                            "pt_l_name": "Johann Christoph von Freyberg-Eisenberg"
                            "pt_t_id": "AUG",
                            "pt_t_kurzform": "AUG",
                            "type": "parent",
                            "my_join_field": {
                                "name": "parent"
                            },
                            "@version": "1",
                            "@timestamp": "2020-01-12T16:45:08.470Z",
                        },
                        "sort": [
                            "AUG"
                        ]
                    }
                ]
            }
        }
    }
}

（实际上，数据字段比示例中的要多，因此我不能使用Decoder.mapX函数。）我可以肯定地知道Decoder.mapX列表中始终只有一个条目/父项，我想将其inner_hits.prnt.hits.hits字段与“ main”对象中的字段一起放平到记录的类型定义中，如下所示：

_source

这里是我到目前为止拥有的解码器：

type alias Child =
    { grp_id : String
    , varia : String
    , pt_archive : String
    , pt_id : String
    , pt_title : String
    , pt_l_id : String
    , pt_l_name : String
    , pt_t_id : String
    , pt_t_shortLabel : String
    }

（[

type alias Parent =
    { pt_archive : String
    , pt_id : String
    , pt_title : String
    , pt_l_id : String
    , pt_l_name : String
    , pt_t_id : String
    , pt_t_shortLabel : String
    }


childHitDecoder : Decoder Child
childHitDecoder =
    Json.Decode.succeed Child
        |> Json.Decode.Pipeline.requiredAt [ "_source", "child_mgrp_id" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "child_varia" ] Json.Decode.string
        |> Json.Decode.Pipeline.custom (Json.Decode.at [ "inner_hits", "prnt", "hits", "hits" ] (firstElementDecoder parentInnerHitDecoder) )


firstElementDecoder : Json.Decode.Decoder a -> Json.Decode.Decoder a
firstElementDecoder baseDecoder =
    Json.Decode.list baseDecoder
        |> Json.Decode.map List.head
        |> Json.Decode.andThen (Maybe.map Json.Decode.succeed >> Maybe.withDefault (Json.Decode.fail "Empty list"))


parentInnerHitDecoder : Decoder Parent
parentInnerHitDecoder =
    Json.Decode.succeed Parent
        |> Json.Decode.Pipeline.requiredAt [ "_source", "archiv" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "id" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "titel" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "lah_id" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "lah_name" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "ter_id" ] Json.Decode.string
        |> Json.Decode.Pipeline.requiredAt [ "_source", "ter_kurzform" ] Json.Decode.string

来自firstElementDecoder，我想用它来取消列出一个inner_hits列表项。]

[使用这样的设置，我的Elm: Decode a JSON array with a single element into a string的最后一行给出了错误：

childHitDecoder

我已经成功地使用

This function cannot handle the argument sent through the (|>) pipe:

482|     Json.Decode.succeed Child
483|         |> Json.Decode.Pipeline.requiredAt [ "_source", "child_mgrp_id" ] Json.Decode.string
484|         |> Json.Decode.Pipeline.requiredAt [ "_source", "child_varia" ] Json.Decode.string
485|         |> Json.Decode.Pipeline.custom (Json.Decode.at [ "inner_hits", "prnt", "hits", "hits" ] (firstElementDecoder parentInnerHitDecoder) )
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The argument is:

    Decoder
        (
        String
        -> String
        -> String
        -> String
        -> String
        -> String
        -> String
        -> Child
        )

But (|>) is piping it to a function that expects:

    Decoder (Parent -> b)

Hint: It looks like it takes too many arguments. I see 6 extra.

来将嵌套的JSON对象解码为其他地方的扁平elm记录，但是显然，将其与此处的除名结合起来已经超出了我。

这使我困惑了好几天，我也尝试过将字段捆绑与Decode.Pipeline.custom组合在一起，但是也失败了。我对Decode.Pipeline.custom或Decode.map的理解不够，无法在这里找到帮助，我将非常感谢您的帮助。

Answer 1

使用上面列出的json对象，以下方法应该起作用：