Question

我正在尝试使用postgres在节点js处跟踪代码以获取嵌套的json对象列表，如下所示：

{
    "elements": [
        {
            "name": "element 1",
            "description": "lorem ipsus",
            "docs": [
                {
                    "docs_id": 1053,
                    "docs_file": "../uploads/1461s444---planimetria.pdf",
                    "docs_created_at": "ThuMay14201506: 00: 00GMT+0200(CEST)"
                },
                {
                    "docs_id": 1054,
                    "docs_file": "../uploads/1461s444---VAX-with-highlight.pdf",
                    "docs_created_at": "ThuMay14201506: 00: 00GMT+0200(CEST)"
                },
                {
                    "docs_id": 1055,
                    "docs_file": "../uploads/1461s444---Elaborato-Planimetrico-with-highlight.pdf",
                    "docs_created_at": "ThuMay14201506: 00: 00GMT+0200(CEST)"
                }
            ]
        },
        {
            "name": "element 2",
            "description": "lorem ipsus",
            "docs": [
                {
                    "docs_id": 1056,
                    "docs_file": "../uploads/pianta.pdf",
                    "docs_created_at": "ThuMay17201106: 00: 00GMT+0200(CEST)"
                },
                {
                    "docs_id": 1055,
                    "docs_file": "../uploads/test.pdf",
                    "docs_created_at": "ThuMay16201706: 00: 00GMT+0200(CEST)"
                }
            ]
        }
    ]
}

使用此代码：

apiRoutes.get('/elements', function(req, res) {

var elements = [];  
knex.select().table('element').where(req.query)
    .then (function(rows){
        for(var i in rows){
            var element =  rows[i];
            knex.select().table('document').where('element_id',rows[i].element_id)
                .then (function(docs){
                    element['docs'] = docs;
                    //console.log (elements);
                    elements.push(element);
                });
        }
        res.json (elements);
    });
});

但结果是一个空数组。

我认为关于查询的异步处理存在问题，但我无法解决。

Answer 1

你的for循环正在进行一堆异步调用。你需要在承诺中归还所有这些。以下是使用bluebird的示例。（我还没有测试过。）

var Promise = require('bluebird')

knex.select().table('element').where(req.query)
    .then (function(rows){

        var promises = rows.map(function(element){
            return knex.select().table('document').where('element_id',element.element_id)
                .then(function(docs){
                    element['docs'] = docs

                    return element;
                });
        })

       return Promise.all(promises)     
    }).then(function(elements){     
        res.json (elements);
    })
});

但是，我认为发出第二个查询是n+1。 Knex支持left outer join。你应该使用它。

Node JS - express - postgres通过循环查询获取嵌套的json对象

1 个答案: