See: Example: Adding tags to a post
TLDNR: For put_assoc to work as expected it needs to be handed full schema structs, not just IDs (plain maps or keyword lists lead to Ecto trying to insert a new record on the other end of the association).
This works without referring to the association table:
# Usually in MusicDB.Album
def changeset(album, params \\ %{}) do
# obtain genres as full schema structs
genres = fetch_selected_genres(params["genres"])
album
|> EC.cast(params, [:title])
|> EC.put_assoc(:genres, genres)
|> EC.validate_required([:title])
end
(I also find that it is usually necessary to show the schemas involved to make sense of some of the changeset and association code)
# file: lib/music_db/artist.ex
defmodule MusicDB.Artist do
use Ecto.Schema
import Ecto.Changeset
alias MusicDB.{Artist, Album}
schema "artists" do
field(:name)
field(:birth_date, :date)
field(:death_date, :date)
timestamps()
has_many(:albums, Album)
has_many(:tracks, through: [:albums, :tracks])
end
def changeset(artist, params) do
artist
|> cast(params, [:name, :birth_date, :death_date])
|> validate_required([:name])
end
def changeset(%MusicDB.Band{} = band) do
{:ok, birth_date} = Date.new(band.year_started, 1, 1)
{:ok, death_date} = Date.new(band.year_ended, 12, 31)
changeset(
%Artist{
name: band.name,
birth_date: birth_date,
death_date: death_date
},
%{}
)
end
def changeset(%MusicDB.SoloArtist{} = solo_artist) do
name =
"#{solo_artist.name1} #{solo_artist.name2} #{solo_artist.name3}"
|> String.trim()
changeset(
%Artist{
name: name,
birth_date: solo_artist.birth_date,
death_date: solo_artist.death_date
},
%{}
)
end
end
# file: lib/music_db/album.ex
defmodule MusicDB.Album do
use Ecto.Schema
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
end
# file: lib/music_db/album_genre.ex
defmodule MusicDB.AlbumGenre do
use Ecto.Schema
alias MusicDB.{Album, Genre}
schema "albums_genres" do
belongs_to(:albums, Album)
belongs_to(:genres, Genre)
end
end
# file: lib/music_db/genre.ex
defmodule MusicDB.Genre do
use Ecto.Schema
alias MusicDB.Album
schema "genres" do
field(:name)
field(:wiki_tag)
timestamps()
many_to_many(:albums, Album, join_through: "albums_genres")
end
end
# file: music_db/priv/repo/playground.exs
#
# http://www.pragmaticprogrammer.com/titles/wmecto
# https://pragprog.com/titles/wmecto/source_code
# http://media.pragprog.com/titles/wmecto/code/wmecto-code.zip
#
# pg_ctl -D /usr/local/var/postgres start
# mix format ./priv/repo/playground.exs
# mix run ./priv/repo/playground.exs
#
defmodule AppInfo do
def string() do
Application.loaded_applications()
|> Enum.map(&to_app_keyword/1)
|> Enum.sort_by(&map_app_name/1)
|> Enum.map_join(", ", &app_keyword_to_string/1)
end
defp to_app_keyword({app, _, vsn}),
do: {app, vsn}
defp app_keyword_to_string({app, vsn}),
do: "#{app}: #{vsn}"
defp map_app_name({app, _}),
do: app
end
defmodule Playground do
import Ecto.Query
alias Ecto.Changeset, as: EC
alias Ecto, as: E
alias MusicDB.Repo
alias MusicDB.{Artist, Album, Genre}
def query_bill_evans do
from(a in Artist, where: a.name == "Bill Evans")
|> Repo.one()
end
def cons_integer(text, values) do
case Integer.parse(text) do
{value, _rest} ->
[value | values]
_ ->
values
end
end
def fetch_selected_genres(selected) do
ids = Enum.reduce(selected, [], &cons_integer/2)
case ids do
[] = empty ->
empty
_ ->
from(g in Genre, where: g.id in ^ids)
|> Repo.all()
end
end
# Usually in MusicDB.Album
def changeset(album, params \\ %{}) do
# obtain genres as full schema structs
genres = fetch_selected_genres(params["genres"])
album
|> EC.cast(params, [:title])
|> EC.put_assoc(:genres, genres)
|> EC.validate_required([:title])
end
def make_params do
%{
"title" => "Test Album",
"genres" => ["1","2"]
}
end
def play do
# Note: Album needs to be associated with
# with an Artist so we use build_assoc
# to create the "shell" schema struct
# for the Album
#
query_bill_evans()
|> E.build_assoc(:albums)
|> changeset(make_params())
|> Repo.insert()
end
end
IO.puts(AppInfo.string())
IO.inspect(Playground.play())
$ mix run ./priv/repo/playground.exs
asn1: 5.0.9, compiler: 7.4.4, connection: 1.0.4, crypto: 4.5.1, db_connection: 2.0.5, decimal: 1.6.0, ecto: 3.0.6, ecto_sql: 3.0.5, elixir: 1.9.1, hex: 0.20.1, inets: 7.0.9, jason: 1.1.2, kernel: 6.4.1, logger: 1.9.1, mariaex: 0.9.1, mix: 1.9.1, music_db: 0.1.0, postgrex: 0.14.1, public_key: 1.6.7, ssl: 9.3.5, stdlib: 3.9.2, telemetry: 0.3.0
18:46:13.559 [debug] QUERY OK source="artists" db=4.4ms decode=0.6ms queue=1.2ms
SELECT a0."id", a0."name", a0."birth_date", a0."death_date", a0."inserted_at", a0."updated_at"
FROM "artists" AS a0 WHERE (a0."name" = 'Bill Evans') []
18:46:13.566 [debug] QUERY OK source="genres" db=0.8ms queue=0.9ms
SELECT g0."id", g0."name", g0."wiki_tag", g0."inserted_at", g0."updated_at"
FROM "genres" AS g0 WHERE (g0."id" = ANY($1)) [[2, 1]]
18:46:13.569 [debug] QUERY OK db=0.2ms
begin []
18:46:13.574 [debug] QUERY OK db=1.2ms
INSERT INTO "albums" ("artist_id","title","inserted_at","updated_at")
VALUES ($1,$2,$3,$4) RETURNING "id" [2, "Test Album", ~N[2019-08-03 22:46:13], ~N[2019-08-03 22:46:13]]
18:46:13.576 [debug] QUERY OK db=0.7ms
INSERT INTO "albums_genres" ("album_id","genre_id")
VALUES ($1,$2) [16, 1]
18:46:13.577 [debug] QUERY OK db=0.2ms
INSERT INTO "albums_genres" ("album_id","genre_id")
VALUES ($1,$2) [16, 2]
18:46:13.583 [debug] QUERY OK db=6.0ms
commit []
{:ok,
%MusicDB.Album{
__meta__: #Ecto.Schema.Metadata<:loaded, "albums">,
artist: #Ecto.Association.NotLoaded<association :artist is not loaded>,
artist_id: 2,
genres: [
%MusicDB.Genre{
__meta__: #Ecto.Schema.Metadata<:loaded, "genres">,
albums: #Ecto.Association.NotLoaded<association :albums is not loaded>,
id: 1,
inserted_at: ~N[2019-08-03 14:35:27],
name: "jazz",
updated_at: ~N[2019-08-03 14:35:27],
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[2019-08-03 14:35:27],
name: "live",
updated_at: ~N[2019-08-03 14:35:27],
wiki_tag: "Concert"
}
],
id: 16,
inserted_at: ~N[2019-08-03 22:46:13],
title: "Test Album",
tracks: #Ecto.Association.NotLoaded<association :tracks is not loaded>,
updated_at: ~N[2019-08-03 22:46:13]
}}
$
