If I have any random number of elements in a list. And I want to get all the elements out of the list .How it can be done
list = [1,2,3,4,5]
member = 1,2,3,4,5
The list size can be changed so Is there any dynamic solution?
I read about hd and tail and index base but they are static for my case.
Thanks
list contains all of its elements, so the list itself is already what you want…
Or could you please clarify what you trying to achieve? Perhaps even try it with a simple example in another language, we have a lot of polyglots here.
I have this query in which I am preloading tables dynamically.
assoc_models = [:example]
from(
q in queryable,
preload: ^assoc_models
)
The result is this:
#Ecto.Query<from j in TestModel.Join, preload: [[:example]]>
There can be any number of tables inside assoc_models
There is an extra pair of brackets after preload that I want to remove.
So you want to go from [[1], [2]] to [1, 2]?
In that case, you can use List.flatten/1:
[[1], [2]]
|> List.flatten()
# => [1, 2]
Check out the documentation of the List and Enum modules. They offer many functions for transforming lists and collection-type data structures.
Why not just assoc = :example; from(q in queryable, preload: ^assoc)?
edit
Oh, wait, I see now, you do not know how many items there will be in assoc_models in advance…
I’d try to use preload-macro then, roughly like this:
assoc = [:foo, :bar]
queryable
|> preload(assoc)
The params are coming in list
[:example]Yeah, I realized it shortly after submitting my answer, please revisit my last post.
Same result. 
And what exactly is the problem with that result - it already works …
#---
# Excerpted from "Programming Ecto",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material,
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose.
# Visit http://www.pragmaticprogrammer.com/titles/wmecto for more book information.
#---
defmodule MusicDB.Album do
use Ecto.Schema
import Ecto.Changeset
alias MusicDB.{Artist, Track, Genre}
schema "albums" do
field(:title, :string)
timestamps()
belongs_to(:artist, Artist)
has_many(:tracks, Track)
many_to_many(:genres, Genre, join_through: "albums_genres")
end
def changeset(album, params) do
album
|> cast(params, [:title])
|> validate_required([:title])
end
end
iex(1)> alias MusicDB.{Repo,Album}
[MusicDB.Repo, MusicDB.Album]
iex(2)> import Ecto.Query
Ecto.Query
iex(3)> album_id = 2
2
iex(4)> assocs = [:tracks, :artist, :genres]
[:tracks, :artist, :genres]
iex(5)> assocs1 = Enum.take(assocs, 1)
[:tracks]
iex(6)> assocs2 = Enum.take(assocs, 2)
[:tracks, :artist]
iex(7)>
nil
iex(8)> query = from(a in Album, [
...(8)> preload: ^assocs1,
...(8)> where: a.id == ^album_id
...(8)> ])
#Ecto.Query<from a in MusicDB.Album, where: a.id == ^2, preload: [[:tracks]]>
iex(9)> Repo.all(query)
09:53:41.711 [debug] QUERY OK source="albums" db=2.4ms decode=1.8ms
SELECT a0."id", a0."title", a0."inserted_at", a0."updated_at", a0."artist_id" FROM "albums" AS a0 WHERE (a0."id" = $1) [2]
09:53:41.718 [debug] QUERY OK source="tracks" db=1.9ms
SELECT t0."id", t0."title", t0."duration", t0."index", t0."number_of_plays", t0."inserted_at", t0."updated_at", t0."album_id", t0."album_id" FROM "tracks" AS t0 WHERE (t0."album_id" = $1) ORDER BY t0."album_id" [2]
[
%MusicDB.Album{
__meta__: #Ecto.Schema.Metadata<:loaded, "albums">,
artist: #Ecto.Association.NotLoaded<association :artist is not loaded>,
artist_id: 1,
genres: #Ecto.Association.NotLoaded<association :genres is not loaded>,
id: 2,
inserted_at: ~N[2018-06-16 20:29:41.476625],
title: "Cookin' At The Plugged Nickel",
tracks: [
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1061,
duration_string: nil,
id: 10,
index: 5,
inserted_at: ~N[2018-06-16 20:29:41.480612],
number_of_plays: 0,
title: "No Blues",
updated_at: ~N[2018-06-16 20:29:41.480618]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 754,
duration_string: nil,
id: 9,
index: 4,
inserted_at: ~N[2018-06-16 20:29:41.480045],
number_of_plays: 0,
title: "Miles",
updated_at: ~N[2018-06-16 20:29:41.480051]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 896,
duration_string: nil,
id: 8,
index: 3,
inserted_at: ~N[2018-06-16 20:29:41.479460],
number_of_plays: 0,
title: "Walkin'",
updated_at: ~N[2018-06-16 20:29:41.479465]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 774,
duration_string: nil,
id: 7,
index: 2,
inserted_at: ~N[2018-06-16 20:29:41.478857],
number_of_plays: 0,
title: "Stella By Starlight",
updated_at: ~N[2018-06-16 20:29:41.478864]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1006,
duration_string: nil,
id: 6,
index: 1,
inserted_at: ~N[2018-06-16 20:29:41.478296],
number_of_plays: 0,
title: "If I Were A Bell",
updated_at: ~N[2018-06-16 20:29:41.478302]
}
],
updated_at: ~N[2018-06-16 20:29:41.476631]
}
]
iex(10)> query = from(a in Album, [
...(10)> preload: ^assocs2,
...(10)> where: a.id == ^album_id
...(10)> ])
#Ecto.Query<from a in MusicDB.Album, where: a.id == ^2,
preload: [[:tracks, :artist]]>
iex(11)> Repo.all(query)
09:53:41.728 [debug] QUERY OK source="albums" db=1.7ms
SELECT a0."id", a0."title", a0."inserted_at", a0."updated_at", a0."artist_id" FROM "albums" AS a0 WHERE (a0."id" = $1) [2]
09:53:41.734 [debug] QUERY OK source="tracks" db=2.0ms
SELECT t0."id", t0."title", t0."duration", t0."index", t0."number_of_plays", t0."inserted_at", t0."updated_at", t0."album_id", t0."album_id" FROM "tracks" AS t0 WHERE (t0."album_id" = $1) ORDER BY t0."album_id" [2]
09:53:41.734 [debug] QUERY OK source="artists" db=1.6ms
SELECT a0."id", a0."name", a0."birth_date", a0."death_date", a0."inserted_at", a0."updated_at", a0."id" FROM "artists" AS a0 WHERE (a0."id" = $1) [1]
[
%MusicDB.Album{
__meta__: #Ecto.Schema.Metadata<:loaded, "albums">,
artist: %MusicDB.Artist{
__meta__: #Ecto.Schema.Metadata<:loaded, "artists">,
albums: #Ecto.Association.NotLoaded<association :albums is not loaded>,
birth_date: nil,
death_date: nil,
id: 1,
inserted_at: ~N[2018-06-16 20:29:41.462600],
name: "Miles Davis",
tracks: #Ecto.Association.NotLoaded<association :tracks is not loaded>,
updated_at: ~N[2018-06-16 20:29:41.462606]
},
artist_id: 1,
genres: #Ecto.Association.NotLoaded<association :genres is not loaded>,
id: 2,
inserted_at: ~N[2018-06-16 20:29:41.476625],
title: "Cookin' At The Plugged Nickel",
tracks: [
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1061,
duration_string: nil,
id: 10,
index: 5,
inserted_at: ~N[2018-06-16 20:29:41.480612],
number_of_plays: 0,
title: "No Blues",
updated_at: ~N[2018-06-16 20:29:41.480618]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 754,
duration_string: nil,
id: 9,
index: 4,
inserted_at: ~N[2018-06-16 20:29:41.480045],
number_of_plays: 0,
title: "Miles",
updated_at: ~N[2018-06-16 20:29:41.480051]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 896,
duration_string: nil,
id: 8,
index: 3,
inserted_at: ~N[2018-06-16 20:29:41.479460],
number_of_plays: 0,
title: "Walkin'",
updated_at: ~N[2018-06-16 20:29:41.479465]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 774,
duration_string: nil,
id: 7,
index: 2,
inserted_at: ~N[2018-06-16 20:29:41.478857],
number_of_plays: 0,
title: "Stella By Starlight",
updated_at: ~N[2018-06-16 20:29:41.478864]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1006,
duration_string: nil,
id: 6,
index: 1,
inserted_at: ~N[2018-06-16 20:29:41.478296],
number_of_plays: 0,
title: "If I Were A Bell",
updated_at: ~N[2018-06-16 20:29:41.478302]
}
],
updated_at: ~N[2018-06-16 20:29:41.476631]
}
]
iex(12)> query = from(a in Album, [
...(12)> preload: ^assocs,
...(12)> where: a.id == ^album_id
...(12)> ])
#Ecto.Query<from a in MusicDB.Album, where: a.id == ^2,
preload: [[:tracks, :artist, :genres]]>
iex(13)> Repo.all(query)
09:53:41.738 [debug] QUERY OK source="albums" db=0.8ms
SELECT a0."id", a0."title", a0."inserted_at", a0."updated_at", a0."artist_id" FROM "albums" AS a0 WHERE (a0."id" = $1) [2]
09:53:41.741 [debug] QUERY OK source="artists" db=1.6ms
SELECT a0."id", a0."name", a0."birth_date", a0."death_date", a0."inserted_at", a0."updated_at", a0."id" FROM "artists" AS a0 WHERE (a0."id" = $1) [1]
09:53:41.741 [debug] QUERY OK source="tracks" db=1.8ms
SELECT t0."id", t0."title", t0."duration", t0."index", t0."number_of_plays", t0."inserted_at", t0."updated_at", t0."album_id", t0."album_id" FROM "tracks" AS t0 WHERE (t0."album_id" = $1) ORDER BY t0."album_id" [2]
09:53:41.744 [debug] QUERY OK source="genres" db=2.4ms
SELECT g0."id", g0."name", g0."wiki_tag", g0."inserted_at", g0."updated_at", a1."id" FROM "genres" AS g0 INNER JOIN "albums" AS a1 ON a1."id" = ANY($1) INNER JOIN "albums_genres" AS a2 ON a2."album_id" = a1."id" WHERE (a2."genre_id" = g0."id") ORDER BY a1."id" [[2]]
[
%MusicDB.Album{
__meta__: #Ecto.Schema.Metadata<:loaded, "albums">,
artist: %MusicDB.Artist{
__meta__: #Ecto.Schema.Metadata<:loaded, "artists">,
albums: #Ecto.Association.NotLoaded<association :albums is not loaded>,
birth_date: nil,
death_date: nil,
id: 1,
inserted_at: ~N[2018-06-16 20:29:41.462600],
name: "Miles Davis",
tracks: #Ecto.Association.NotLoaded<association :tracks is not loaded>,
updated_at: ~N[2018-06-16 20:29:41.462606]
},
artist_id: 1,
genres: [
%MusicDB.Genre{
__meta__: #Ecto.Schema.Metadata<:loaded, "genres">,
albums: #Ecto.Association.NotLoaded<association :albums is not loaded>,
id: 1,
inserted_at: ~N[2018-06-16 20:29:41.452406],
name: "jazz",
updated_at: ~N[2018-06-16 20:29:41.452415],
wiki_tag: "Jazz"
},
%MusicDB.Genre{
__meta__: #Ecto.Schema.Metadata<:loaded, "genres">,
albums: #Ecto.Association.NotLoaded<association :albums is not loaded>,
id: 2,
inserted_at: ~N[2018-06-16 20:29:41.460689],
name: "live",
updated_at: ~N[2018-06-16 20:29:41.460696],
wiki_tag: "Concert"
}
],
id: 2,
inserted_at: ~N[2018-06-16 20:29:41.476625],
title: "Cookin' At The Plugged Nickel",
tracks: [
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1061,
duration_string: nil,
id: 10,
index: 5,
inserted_at: ~N[2018-06-16 20:29:41.480612],
number_of_plays: 0,
title: "No Blues",
updated_at: ~N[2018-06-16 20:29:41.480618]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 754,
duration_string: nil,
id: 9,
index: 4,
inserted_at: ~N[2018-06-16 20:29:41.480045],
number_of_plays: 0,
title: "Miles",
updated_at: ~N[2018-06-16 20:29:41.480051]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 896,
duration_string: nil,
id: 8,
index: 3,
inserted_at: ~N[2018-06-16 20:29:41.479460],
number_of_plays: 0,
title: "Walkin'",
updated_at: ~N[2018-06-16 20:29:41.479465]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 774,
duration_string: nil,
id: 7,
index: 2,
inserted_at: ~N[2018-06-16 20:29:41.478857],
number_of_plays: 0,
title: "Stella By Starlight",
updated_at: ~N[2018-06-16 20:29:41.478864]
},
%MusicDB.Track{
__meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
album: #Ecto.Association.NotLoaded<association :album is not loaded>,
album_id: 2,
duration: 1006,
duration_string: nil,
id: 6,
index: 1,
inserted_at: ~N[2018-06-16 20:29:41.478296],
number_of_plays: 0,
title: "If I Were A Bell",
updated_at: ~N[2018-06-16 20:29:41.478302]
}
],
updated_at: ~N[2018-06-16 20:29:41.476631]
}
]
iex(14)>
I need some sleep …
Try:
(from product in Product, preload: ^assoc) |> Repo.one
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