How to get struct in array

shahryarjb · May 1, 2018, 2:32pm

Hello, how do I get struct in array ? please see my pic :

I need to get the struct’s params , like this : a.cms_post.title

My output is :

[
  %TrangellCmsService.Cms.Db.Post{
    __meta__: #Ecto.Schema.Metadata<:loaded, "cms_post">,
    changelog: true,
    changelog_category: "/ss/ppos",
    cms_post_category: #Ecto.Association.NotLoaded<association :cms_post_category is not loaded>,
    cms_post_category_id: "960c30f2-183e-4430-a96d-4f0fd41fe37b",
    description: "test test test test test test tetststststss tskkkstest of test fo test of test more test more test for test for test tes",
    discourse: true,
    discourse_link: "/ss/ppos",
    download_ext_link: "/ss/ppos", 
    group_acl: "admin",
    id: 6,
    inserted_at: ~N[2018-04-29 11:45:05.311436],
    learn: true,
    learn_category: "/ss/ppos",
    pic_x1_link: "/ss/ppos",
    pic_x2_link: "/ss/ppos",
    pic_x3_link: "/ss/ppos",
    plugin: true,
    plugin_category: "/ss/ppos",
    post_type: "free",
    price: "30000",
    screen_shot: false,
    screen_shot_category: "/ss/ppos",
    seo_alias_link: "7seo-why-seo",
    seo_description: "for the test for test for test for test tete of test",
    seo_language: "fa",
    seo_language_link: "/ss/ppos",
    seo_tag: "test,test,test,test",
    seo_words: "test,test,test,test",
    status: true,
    title: "shahryar",
    updated_at: ~N[2018-04-29 11:45:05.311884]
  }
]

NobbZ · May 1, 2018, 2:45pm

Not array, it is a linked list, and you can use the usual iteration functions from the Enum module.

You can not directly acces the fields because it’s ambiguous which of the potentially many lists contents you want to access.

shahryarjb · May 1, 2018, 3:14pm

Do you prefer pattern or Enum? for example :

I created func like this :

def list_struct_converter(list_params) do
   [%TrangellCmsService.Cms.Db.Post{} = o | _c ] = list_params
   o
end

Now , I get this like this way :

%{ title: list_struct_converter(p.cms_post).title }

by the way, although my problem is solved, but I couldn’t find correct Enum to fix my problem.

peerreynders · May 1, 2018, 3:51pm

Something like

Enum.map(a.cms_post, &(Map.get(&1,:title)))

would return a list of title values

Map.get/3
Enum.map/2

a.cms_post
|> hd()
|> Map.get(:title)

returns the title of the post in the head of the list

Kernel.hd/1

def get_title([%TrangellCmsService.Cms.Db.Post{title: title}|_]),
  do: title

i.e. using pattern matching for destructuring rather than dot notation.

shahryarjb · May 3, 2018, 6:59am

Hello @peerreynders , thank you ,

I was trying very much, but I couldn’t use Map,Enum in my original outputs

my original outputs : https://gist.github.com/shahryarjb/8cd57c2f76f3b43de970c331bbbe59f9

the post variable which I linked in github gist

	def load_posts_by_category_alias(category_alias, group_acl) do
		post = fetch_posts_by_category_alias(category_alias, group_acl)

		for p <- post do
			Enum.map(p, &(Map.get(&1,:title)))
		end
	end

Eiji · May 3, 2018, 7:41am

@shahryarjb If I understand you correctly then this code should help you:

categories # here you have root list from your gist
|> Enum.find(& &1.seo_alias_link == "jsjsjs") # here you have list element
|> Map.fetch!(:cms_post) # here you are fetching sub-list from element
|> Enum.map(& &1.title) # finally here you are getting title for all elements

shahryarjb · May 3, 2018, 8:17am

Hello @Eiji , I edited the code to improve my project , I think my way isn’t right, Do you think like me?

def load_posts_by_category_alias(category_alias, group_acl) do
		category = fetch_posts_by_category_alias(category_alias, group_acl)
		|> Enum.find(& &1.seo_alias_link == "#{category_alias}")

		post = category
		|> Map.fetch!(:cms_post)
		
		for %TrangellCmsService.Cms.Db.Post{} = p <- post do
			%{post_id: p.id, category_title: category.title}
		end
	end

Eiji · May 3, 2018, 8:20am

@shahryarjb: Sorry, but I don’t know few things:

What’s value of category_alias variable - "jsjsjs" ?
What’s value of group_acl variable - root list from gist ?
fetch_posts_by_category_alias function body

If I assumed correctly (about your variables) then:

def load_posts_by_category_alias(category_alias, group_acl) do
  group_acl
  |> Enum.find(& &1.seo_alias_link == category_alias)
  |> Map.fetch!(:cms_post)
  |> Enum.map(& &1.title)
end

shahryarjb · May 3, 2018, 8:48am

I am sorry for this and thank you again,

the Category_alias is unique slug link which indexed in my database, for this purpose my client searches with it. and group_acl is user’s access,

At the end, fetch_posts_by_category_alias is :

	defp fetch_posts_by_category_alias(seo_alias_link, "admin") do
	  Repo.all from p in PostCategory,
	           join: c in assoc(p, :cms_post),
	           where: p.seo_alias_link == ^seo_alias_link,
	           order_by: p.inserted_at,
	           limit: 10,
	           preload: [cms_post: c]
	end

	defp fetch_posts_by_category_alias(seo_alias_link, group_acl) do
	  Repo.all from p in PostCategory,
	           join: c in assoc(p, :cms_post),
	           where: p.seo_alias_link == ^seo_alias_link,
	           where: p.group_acl == ^group_acl,
	           where: c.group_acl == ^group_acl,
	           order_by: p.inserted_at,
	           limit: 10,
	           preload: [cms_post: c]
	end

My problem started since I used preload , preload doesn’t let me filter my output at first.

my all codes :

Eiji · May 3, 2018, 9:24am

@shahryarjb: Then it’s really simple. Assuming you want only list of titles:

defmodule Example do
  import Ecto.Query

  # aliases for Post and PostCategory here

  def sample(seo_alias_link, group_acl) do
    query =
      from(
        post_category in PostCategory,
        inner_join: post in assoc(post_category, :cms_post),
        order_by: post_category.inserted_at,
        limit: 10,
        select: post.title,
        where: p.seo_alias_link == ^seo_alias_link
      )

    if group_acl == "admin" do
      query
    else
      where(
        query,
        [post_category, post],
        post_category.group_acl == ^group_acl and post.group_acl == ^group_acl
      )
    end
  end
end

"jsjsjs" |> Example.sample("admin") |> Repo.all

shahryarjb · May 3, 2018, 9:32am

I know this , but I want to use preload, without preload is simple and very clear, so why was preload made , really? and where I use this ?

Eiji · May 3, 2018, 9:52am

@shahryarjb: I assumed you want to fetch online titles. If so then easier is to call specific query.

defmodule Example do
  import Ecto.Query

  # aliases for Post and PostCategory here

  def sample(seo_alias_link, group_acl) do
    query =
      from(
        post_category in PostCategory,
        join: post in assoc(post_category, :cms_post)
        order_by: post_category.inserted_at,
        limit: 10,
        preload: [cms_post: post],
        where: p.seo_alias_link == ^seo_alias_link
      )

    if group_acl == "admin" do
      query
    else
      where(
        query,
        [post_category, post],
        post_category.group_acl == ^group_acl and post.group_acl == ^group_acl
      )
    end

    query
    |> Repo.all()
    |> Enum.map(& &1.cms_post)
    |> List.flatten()
    |> Enum.map(& &1.title)
  end
end

Example.sample("jsjsjs", "admin")

shahryarjb · May 3, 2018, 10:11am

What does it have effect in performance between regular join and preload? just preload is easier or something more?

Thank you for giving me your time.

Eiji · May 3, 2018, 10:15am

@shahryarjb: In my previous answer &Example.sample/2 returns query. Calling &Repo.all/1 on such result should return already list of titles, so you do not need to work on lists and structs to fetch them manually. I think that such query-only way is easier.