Elixir ecto 2创建many_to_many关联 [英] Elixir ecto 2 create many_to_many association

问题描述

我如何与ecto 2建立多对多关系?作为示例应用程序，我想 创建可以属于多个类别的帖子.类别已经存在.例如:

How can I make a many to many relation with ecto 2? As an example app I want to create a Post which can be in multiple categories. The categories already exist. For example:

[%Category{id: "1", name: "elixir"}, %Category{id: "2", name: "erlang"}]

Im使用Ecto 2 beta0.示例项目称为Ecto2.

Im using Ecto 2 beta 0. The example project is called Ecto2.

我定义了两个模型:

defmodule Ecto2.Post do
  use Ecto2.Web, :model
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    many_to_many :categories, Ecto2.Category, join_through: "posts_categories", on_replace: :delete
    timestamps
  end

  @required_fields ~w(title)
  @optional_fields ~w()
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:categories)  # not suitable?
  end
end

defmodule Ecto2.Category do
  use Ecto2.Web, :model

  schema "categories" do
    field :name, :string

    timestamps
  end

  @required_fields ~w(name)
  @optional_fields ~w()
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
  end
end

我试图这样做:

post = Repo.get!(Post, 1) |> Repo.preload(:categories)
changeset = Post.changeset(post, %{"title"=> "bla", "categories"=> [%{id: "1"}]})
Repo.update!(changeset)

但是Post.changeset中的cast_assoc不适合此任务，它想创建一个全新的Category而不是associate. 我应该怎么用呢? build_assoc?但是build_assoc文档没有提到它对many_to_many很有用.如何使用?那么我应该将build_assoc放在Post.changeset中，还是应该在phoenix控制器中使用它.

But cast_assoc in Post.changeset is not suitable for this task, it wants to create a whole new Category instead of associate one. What should I use instead? build_assoc? But build_assoc docs do not mention it is useful with many_to_many. How do I use it? Should I put build_assoc in Post.changeset then, or should i use it in a phoenix controller.

推荐答案

您可以通过传递诸如"posts_categories"之类的字符串来通过表进行联接，也可以通过诸如MyApp.PostCategory之类的方案来通过一种方案进行联接.我更喜欢通过模式加入，因为可以包含时间戳.假设您选择通过模式而不是表进行连接:

You can join through a table by passing a string like "posts_categories", or through a schema by passing through a schema like MyApp.PostCategory. I prefer joining through schema as timestamps can be included. Let say you choose join through a schema instead of a table:

1. 您需要为要加入的many_to_many关系创建一个单独的表(例如:posts_categories).

```

def change do
  create table(:posts_categories) do
    add :post_id, references(:posts)
    add :category_id, references(:categories)
    timestamps
  end
end

1. 为在步骤1中创建的表创建架构.在web \ models文件夹中，创建文件post_category.ex:

```

defmodule Ecto2.PostCategory do
use Ecto2.Web, :model

schema "posts_categories" do
  belongs_to :post, Ecto2.Post
  belongs_to :category, Ecto2.Category
  timestamps
end

def changeset(model, params \\ %{}) do
  model
  |> cast(params, [])
end
end

Ecto beta 2已将:empty更改为空映射，并将cast \ 4更改为cast \ 3.检查变更日志.

Ecto beta 2 has changed :empty to empty map and change cast\4 to cast \3. Check changelog.

1. 将此行添加到您的帖子架构中:

1. Add this line to your post schema:

many_to_many :categories, Ecto2.Category, join_through: Ecto2.PostCategory

many_to_many :categories, Ecto2.Category, join_through: Ecto2.PostCategory

将此行添加到您的类别架构:

Add this line to your category schema:

many_to_many :posts, Ecto2.Post, join_through: Ecto2.PostCategory

many_to_many :posts, Ecto2.Post, join_through: Ecto2.PostCategory

就是这样！现在您可以像这样更新 ```

That's it! Now you can update like ```

post1 = Repo.get!(Post, 1)
category1 = Repo.get!(Category, 1)

post1
|> Repo.preload(:categories)
|> Post.changeset(%{})
|> put_assoc(:categories, [category1])
|> Repo.update!

```

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