Elastic cannot find nested element

This is the mapping:

    {
        myindex: {
            mappings: {
                properties: {
                    identifier: {
                        type: "keyword"
                    },
                    parts: {
                        type: "nested",
                        properties: {
                            _id: {
                                type: "text",
                                fields: {
                                    keyword: {
                                        type: "keyword",
                                        ignore_above: 256
                                    }
                                }
                            },
                            user_id: {
                                type: "text",
                                fields: {
                                    keyword: {
                                        type: "keyword",
                                        ignore_above: 256
                                    }
                                }
                            }
                        }   
                    }
                }
            }
        }
    }       

The index contains this entry:

_source: {
    identifier: "6002af6aa8672d4f8d06be3b6002c4db0fa23460882e82ab",
    parts: [
        {
            _id: "6002c4dc0fa23460882e82af",
            user_id: "6002c4db0fa23460882e82ab",
        },
        {
            _id: "6002c4dc0fa23460882e82b0",
            user_id: "6002af6aa8672d4f8d06be3b",
        }
    ]
}

When searching for a top level field, then the entry above is returned:

curl -X GET "localhost:9200/myindex/_search?pretty" -H 'Content-Type: application/json' -d'
{
    "query": {
        "bool": {
            "must": [
                {
                    "term": {
                        "identifier": "6002af6aa8672d4f8d06be3b6002c4db0fa23460882e82ab"
                    }
                }
            ]
        }
    }
}'  

However, when I run this query (for searching inside the "parts" array), I don't get any result (i.e. an empty hits):

curl -X GET "localhost:9200/myindex/_search?pretty" -H 'Content-Type: application/json' -d'
{
    "query": {
        "bool": {
            "must": [
                {
                    "term": {
                        "parts.user_id": "6002c4db0fa23460882e82ab"
                    }
                }
            ]
        }
    }
}'

Why? I read about nested elements. What I understood, mapping the field named "parts" as nested is the right way to search inside arrays.