Ecto Without a DB: Many Meanings of Map Greg Vaughn 

• @gvaughn or @gregvaughn on GitHub, Twitter, Slack, ElixirForum, etc. • Elixiring since 2013 • Professionally since 2017 Who Dat 

ORM? ORM? ORM What's Ecto? 

Main Parts of Ecto ✔ ✔ 

iex environment $ mix phx.new ecto_without_a_db --module EctoNoDb --no-ecto Added to mix.exs in `deps/0` function: {:ecto, "~> 3.0"}, {:phoenix_ecto, "~> 4.0"} $ mix deps.get $ iex -S mix 

Structs are maps with superpowers Every journey starts with a map 

Anatomy of a Struct iex(1)> defmodule A, do: defstruct [a: 1, b: 2, c: 3] {:module, A, <<... iex(2)> inspect %A{} "%A{a: 1, b: 2, c: 3}" iex(3)> inspect %A{}, structs: false "%{__struct__: A, a: 1, b: 2, c: 3}" iex(4)> A.__info__(:functions) [__struct__: 0, __struct__: 1] iex(5)> inspect A.__struct__(), structs: false "%{__struct__: A, a: 1, b: 2, c: 3}" 

Schemas are Structs with superpowers2 

Anatomy of an Embedded Schema iex(1)> defmodule B do ...(1)> use Ecto.Schema; embedded_schema do ...(1)> field(:a, :integer); field(:b, :string); field(:c, :float) ...(1)> end ...(1)> end {:module, B, <<... iex(2)> inspect %B{}, structs: false "%{__struct__: B, a: nil, b: nil, c: nil, id: nil}" iex(3)> B.__info__(:functions) [__changeset__: 0, __schema__: 1, __schema__: 2, __struct__: 0, __struct__: 1] iex(4)> B.__schema__(:fields) [:id, :a, :b, :c] iex(5)> B.__changeset__() %{a: :integer, b: :string, c: :float, id: :binary_id} 

Changesets are Command Objects Changesets are Command Objects Structs 

Anatomy of a Changeset iex(1)> inspect %Ecto.Changeset{} #Ecto.Changeset< action: nil, changes: %{}, errors: [], data: nil, valid?: false > iex(2)> inspect %Ecto.Changeset{}, structs: false "%{__struct__: Ecto.Changeset, action: nil, changes: %{}, constraints: [], data: nil, empty_values: [\"\"], errors: [], filters: %{}, params: nil, prepare: [], repo: nil, repo_opts: [], required: [], types: nil, valid?: false, validations: [] }" 

Anatomy of a Changeset iex(3)> c = Ecto.Changeset.cast(%B{a: 0}, %{a: 1, b: "Lonestar", c: 3.0}, [:a, :b]) #Ecto.Changeset< action: nil, changes: %{a: 1, b: "Lonestar"}, errors: [], data: #B<>, valid?: true > iex(4)> inspect c, structs: false "%{__struct__: Ecto.Changeset, action: nil, changes: %{a: 1, b: \"Lonestar\"}, constraints: [], empty_values: [\"\"], errors: [], filters: %{}, prepare: [], repo: nil, repo_opts: [], required: [], valid?: true, validations: [] data: %{__struct__: B, a: 0, b: nil, c: nil, id: nil}, params: %{\"a\" => 1, \"b\" => \"Lonestar\", \"c\" => 3.0}, types: %{a: :integer, b: :string, c: :float, id: :binary_id}, }" 

Back to Struct-land iex(1)> {:ok, b} = Ecto.Changeset.apply_action(c, :update) {:ok, %B{a: 1, b: "Lonestar", c: nil, id: nil}} iex(2)> Ecto.Changeset.cast(%B{}, %{a: 3.14}, [:a, :b]) |> ...(2)> Ecto.Changeset.apply_action(:update) {:error, #Ecto.Changeset< action: :update, changes: %{}, errors: [a: {"is invalid", [type: :integer, validation: :cast]}], data: #B<>, valid?: false >} 

Elmo loves "M" for "mapping!" Elmo loves Ecto because it rhymes with "Elmo!" 

The Phoenix Protocols 

Transform to Form iex(1)> {:error, c} = Ecto.Changeset.cast(%B{}, %{a: 3.14}, [:a, :b]) |> Ecto.Changeset.apply_action(:update) ... iex(2)> Phoenix.HTML.Form.form_for(c, "http://somewhere/clever") %Phoenix.HTML.Form{ action: "http://somewhere/clever", data: %B{a: nil, b: nil, c: nil, id: nil}, errors: [a: {"is invalid", [type: :integer, validation: :cast]}], hidden: [], id: "b", impl: Phoenix.HTML.FormData.Ecto.Changeset, index: nil, name: "b", options: [method: "post"], params: %{"a" => 3.14}, source: #Ecto.Changeset< action: :update, changes: %{}, errors: [a: {"is invalid", [type: :integer, validation: :cast]}], data: #B<>, valid?: false > } 

Example: Advanced Search defmodule EctoNoDb.EventSearchController do use EctoNoDbWeb, :controller def index(conn, %{"q" => query_params}) do ... end end 

Example: Advanced Search iex(1)> query_params = %{"name" => "BBQ", "city" => "Austin", "start" => "2019-02-28", "end" => "2019-03-03"} iex(2)> types = %{name: :string, city: :string, start: :date, end: :date} iex(3)> c = Ecto.Changeset.cast({%{}, types}, query_params, Map.keys(types)) #Ecto.Changeset< action: nil, changes: %{ city: "Austin", end: ~D[2019-03-03], name: "BBQ", start: ~D[2019-02-28] }, errors: [], data: %{}, valid?: true > 

Example: Advanced Search iex(4)> inspect c, structs: false "%{__struct__: Ecto.Changeset, ... without fields shown above ... constraints: [], empty_values: [\"\"], filters: %{}, prepare: [], repo: nil, repo_opts: [], required: [], validations: [], params: %{\"city\" => \"Austin\", \"end\" => \"2019-03-03\", \"name\" => \"BBQ\", \"start\" => \"2019-02-28\"}, types: %{city: :string, end: :date, name: :string, start: :date} }" iex(5)> Ecto.Changeset.apply_action(c, :update) {:ok, %{city: "Austin", end: ~D[2019-03-03], name: "BBQ", start: ~D[2019-02-28]}} 

Example: Advanced Search defmodule EctoNoDb.EventSearchController do use EctoNoDbWeb, :controller @query_types = %{name: :string, city: :string, start: :date, end: :date} def index(conn, %{"q" => query_params}) do with {:ok, query} <- cast_params(query_params, @query_types), {:ok, results} <- NotInScopeForThisTalk.get_results(query) do ... do your thang ... end end defp cast_params(query_params, type_map) do {%{}, type_map} |> Ecto.Changeset.cast(query_params, Map.keys(type_map)) |> Ecto.Changeset.apply_action(:update) end end 

Anti-Corruption Layer “If your application needs to deal with a database or another application whose model is undesirable or inapplicable to the model you want within your own application, use an AnticorruptionLayer to translate to/from that model and yours.” – c2 wiki 

Anti-Corruption Layer API Client Anti Corruption Layer Domain Logic Typed Struct 

Anti-Corruption Layer defmodule EctoNoDb.Event do use Ecto.Schema import Ecto.Changeset embedded_schema do field(:name, :string) field(:venue_name, :string) field(:venue_city, :string) field(:occurs_at, :naive_datetime) end def new(raw_map) do %__MODULE__{} |> cast(raw_map, [:name, :venue_name, :venue_city, :occurs_at]) |> apply_action(:update) end end 

Anti-Corruption Layer defmodule EctoNoDb.Event do use Ecto.Schema import Ecto.Changeset embedded_schema do field(:name, :string) field(:occurs_at, :naive_datetime) embeds_one(:venue, EctoNoDb.Venue) embeds_one(:performer, EctoNoDb.Performer) end def new(raw_map) do %__MODULE__{} |> cast(raw_map, [:name, :occurs_at]) |> cast_embed(:venue) |> cast_embed(:performer) |> apply_action(:update) end end 

Anti-Corruption Layer defmodule EctoNoDb.Event do ... def new(raw_map) do %__MODULE__{} |> changeset(raw_map) |> apply_action(:update) end defp changeset(base, raw_map) do base |> cast(raw_map, [:name, :occurs_at]) |> cast_embed(:venue) |> cast_embed(:performer) end ... end 

Meta-Anti-Corruption Layer defmodule EctoNoDb.Event do use EctoNoDb.AntiCorruptionSchema embedded_schema do field(:name, :string) field(:occurs_at, :naive_datetime) embeds_one(:venue, EctoNoDb.Venue) embeds_one(:performer, EctoNoDb.Performer) end end defmodule EctoNoDb.Venue do use EctoNoDb.AntiCorruptionSchema embedded_schema do field(:name, :string) field(:city, :string) end end 

Meta-Anti-Corruption Layer defmodule EctoNoDb.AntiCorruptionSchema do defmacro __using__(_opts) do quote do use Ecto.Schema @primary_key false def new(raw_map), do: unquote(__MODULE__).generic_new(raw_map, __MODULE__) def changeset(base, params), do: unquote(__MODULE__).generic_changeset(base, params) defoverridable new: 1, changeset: 2 end end def generic_new(raw_map, struct_module) do struct(struct_module) |> struct_module.changeset(raw_map) |> Ecto.Changeset.apply_action(:update) end def generic_changeset(base, raw_map) do struct_mod = base.__struct__ embeds = struct_mod.__schema__(:embeds) allowed = struct_mod.__schema__(:fields) -- embeds chg = Ecto.Changeset.cast(base, raw_map, allowed) Enum.reduce(embeds, chg, &Ecto.Changeset.cast_embed(&2, &1)) end end 

Thank You • Takeaways • Maps -> Structs -> Schemas -> Changesets -> Structs/Maps • Understanding one level below where you are focused • Ideas for when to use typed structs in your domain • [email protected] • Twitter: @gregvaughn • Elixir Slack: @gregvaughn • Elixir Forum: gregvaughn • Questions?