为什么MongoDB填充方法在此应用程序中失败? [英] Why does MongoDB populate method fail in this application?

问题描述

我正在使用 博客应用程序 (点击指向请参见 GitHub 存储库)，其中 Express ，

I am working on a blogging application (click the link to see the GitHub repo) with Express, EJS and MongoDB.

我有帖子和帖子类别，它们都在自己的收藏集中.

I have Posts and Post Categories, each in its own collection.

类别架构:

const mongoose = require('mongoose');

const categorySchema = new mongoose.Schema({
    cat_name: {
        type: String,
        required: true
    },
    updated_at: {
        type: Date,
        default: Date.now()
    },
    created_at: {
        type: Date,
        default: Date.now()
    }
});

module.exports = mongoose.model('Category', categorySchema);

帖子架构:

const mongoose = require('mongoose');

const postSchema = new mongoose.Schema({
    title: {
        type: String,
        required: true
    },
    short_description: {
        type: String,
        required: true
    },
    full_text: {
        type: String,
        required: true
    },
    category: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'category'
    },
    post_image: {
        type: String,
        required: false
    },
    updated_at: {
        type: Date,
        default: Date.now()
    },
    created_at: {
        type: Date,
        default: Date.now()
    }
});

module.exports = mongoose.model('Post', postSchema);

显示帖子的控制器方法:

exports.displayDashboard = (req, res, next) => {
    const posts = Post.find({}, (err, posts) => {
        if(err){
            console.log('Error: ', err);
        } else {
            res.render('admin/index', {
              layout: 'admin/layout',
              website_name: 'MEAN Blog',
              page_heading: 'Dashboard',
              posts: posts
            });
        }
    }).populate({ path: 'category', model: Category })
};

如果在视图中我有:

<table class="table table-striped table-sm mb-0">
    <thead>
        <tr class="d-flex">
            <th class="col-1 pl-2">#</th>
            <th class="col-2">Title</th>
            <th class="col-2">Category</th>
            <th class="col-2">Created</th>
            <th class="col-2">Updated</th>
            <th class="col-3 pr-2 text-right">Actions</th>
        </tr>
    </thead>
    <tbody>
        <% if (posts) { %>
            <% posts.forEach(function(post, index) { %>
                <tr data-id="<%= post._id %>" class="d-flex">
                    <td class="col-1 pl-2">
                        <%= index + 1 %>
                    </td>
                    <td class="col-2">
                        <a href="/<%= post._id %>" class="text-dark">
                            <%= post.title %>
                        </a>
                    </td>
                    <td class="col-2">
                        <%= post.category %>
                    </td>
                    <td class="col-2">
                        <%= post.created_at.toDateString(); %>
                    </td>
                    <td class="col-2">
                        <%= post.updated_at.toDateString(); %>
                    </td>
                    <td class="col-3 text-right pr-2">
                        <div class="btn-group">
                            <a href="../dashboard/post/edit/<%= post._id %>" class="btn btn-sm btn-success">Edit</a>
                            <a href="#" class="btn btn-sm btn-danger delete-post" data-id="<%= post._id %>">Delete</a>
                        </div>
                    </td>
                </tr>
                <% }); %>
                <% } else { %>
                <tr>
                  <td colspan="5">There are no posts</td>
                </tr>
                <% } %>
    </tbody>
</table>

一个对象显示在帖子表的类别列中:

an object is displayed in The Category column (of the posts table):

但是，如果我尝试使用<%= post.category.cat_name %>显示类别名称，则会出现此错误:

Yet, ig I try to display the category name using <%= post.category.cat_name %> I get this error:

Cannot read property 'cat_name' of null

我在做什么错了?

推荐答案

我发现了问题所在:我已经删除了帖子的先前分配的类别.修复很简单(在视图中):

I found out what the problem was: I had deleted a category previously assigned to a post. The fix is simple (in the view):

<%= post.category != null ? post.category.cat_name : 'No category assigned' %>

代替

<%= post.category.cat_name %>

非常感谢参与其中的每个人.

Many thanks to everyone that got involved for the help.

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