How to get struct in array

@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.

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.

@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")

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 ?

@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

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 :

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

1. What’s value of category_alias variable - "jsjsjs" ?
2. What’s value of group_acl variable - root list from gist ?
3. 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

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

@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

Hello @peerreynders , thank you ,

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

my original outputs : data.ex · GitHub

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