Laravel 显示带有数据透视表错误的书籍类别

Question

我正在学习 Laravel，我有两个表格书籍和类别 我创建了一个带有 book_id 和 category_id 列的数据透视表来建立表之间的关系，但是当我尝试在 show book 页面中显示类别并且我在 $books->categories 上进行 foreach 时，我遇到了一个错误，说foreach 参数为空

Book 模型文件：

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Book extends Model
{
    use HasFactory;

    protected $fillable = ['title', 'desc', 'img'];

    public function categories()
    {
        return $this->belongsTo('App\Models\Category');
    }
}

类别模型文件

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    use HasFactory;
    
    protected $fillable = ['name'];

    public function books()
    {
        return $this->belongsTo(Book::class)->withPivot('book_category'); 
    }
}

show.blade 页面：

@extends('layout')

@section('title')
    Book #{{ $book->id }}
@endsection

@section('content')
    <h2>{{ $book->title }}</h2>
    <p>{{ $book->desc }}</p>
    @foreach ($book->categories as $category)
        <span class="text-muted mx-3">{{ $category->name }}</span>
    @endforeach
    <div class="row">
        <div class="col-md-3 p-5">
            <img src="{{ asset('uploads/books') }}/{{$book->img}}" class="w-100">
        </div>
    </div>
    <a href="{{ route('books.index') }}" class="btn btn-primary">Back</a>
    <a href="{{ route('books.edit', $book->id) }}" class="btn btn-warning mx-2">Edit</a>
    <a href="{{ route('books.delete', $book->id) }}" class="btn btn-danger">Delete</a>
@endsection

数据透视表：

the pivot table

BookController 代码：

public function show($id)
    {
        $book = Book::findOrFail($id);

        return view('books/show', compact('book'));
    }

Answer 1

您可以更改与 belongsToMany 的关系，因为您使用的是数据透视表。

在图书模型中

public function categories()
{
    return $this->belongsToMany(Category::class);
}

在Category 模型中

  public function books()
  {
        return $this->belongsToMany(Book::class); 
  }

在你的控制器中

  $book = Book::with('categories')->findOrFail($id);

在你看来是这样

{{$book->$title}}
@foreach ($book->categories as $category)
    <span class="text-muted mx-3">{{ $category->name }}</span>
@endforeach