some logic added

README.md

@@ -1,5 +1,13 @@
 # PaperTrail
 
+```mix papertrail.install```
+
+PaperTrail.create/1, PaperTrail.update/1, PaperTrail.destroy/1
+
+PaperTrail.get_version\2, PaperTrail.get_version\1 PaperTrail.get_versions\2, PaperTrail.get_versions\1
+
+I will write some tests for this library.
+
 **TODO: Add description**
 
 ## Installation
@@ -17,4 +25,3 @@ If [available in Hex](https://hex.pm/docs/publish), the package can be installed
         def application do
           [applications: [:paper_trail]]
         end
-

lib/paper_trail.ex

@@ -1,2 +1,92 @@
 defmodule PaperTrail do
+  alias Ecto.Multi
+  import Ecto.Query
+  alias Model.Version
+
+  def get_versions(model, id) do
+    item_type = model |> Module.split |> List.last
+    version_query(item_type, id) |> Application.Repo.all
+  end
+
+  def get_versions(changeset) do
+    item_type = changeset.__struct__ |> Module.split |> List.last
+    version_query(item_type, changeset.id) |> Application.Repo.all # where id, item_type last inserted_at
+  end
+
+  def get_version(model, id) do
+    item_type = Module.split(model) |> List.last
+    version_query(item_type, id) |> Application.Repo.one
+  end
+
+  def get_version(changeset) do
+    item_type = changeset.__struct__ |> Module.split |> List.last
+    last(version_query(item_type, changeset.id)) |> Application.Repo.one # where id, item_type last inserted_at
+  end
+
+  defp version_query(item_type, id) do
+    from v in Version,
+    where: v.item_type == ^item_type and v.item_id == ^id
+  end
+
+  # changeset = Model.changeset(Ecto.Repo.get(Model, id), params)
+
+  def insert(struct, meta \\ nil) do
+    Multi.new
+    |> Multi.insert(:model, struct)
+    |> Multi.run(:version, fn %{model: model} ->
+        version = make_version_struct(%{event: "create"}, model, meta)
+        Application.Repo.insert(version)
+      end)
+    |> Application.Repo.transaction
+  end
+
+  def update(changeset, meta \\ nil) do
+    Multi.new
+    |> Multi.update(:model, changeset)
+    |> Multi.run(:version, fn %{model: model} ->
+        version = make_version_struct(%{event: "update"}, changeset, meta)
+        Application.Repo.insert(version)
+      end)
+    |> Application.Repo.transaction
+  end
+
+  def delete(struct, meta \\ nil) do
+    Multi.new
+    |> Multi.delete(:model, struct)
+    |> Multi.run(:version, fn %{model: model} ->
+        version = make_version_struct(%{event: "destroy"}, model, meta)
+        Application.Repo.insert(version)
+      end)
+    |> Application.Repo.transaction
+  end
+
+  def make_version_struct(%{event: "create"}, model, meta) do
+    %Version{
+      event: "create",
+      item_type: model.__struct__ |> Module.split |> List.last,
+      item_id: model.id,
+      item_changes: Map.drop(model, [:__struct__, :__meta__]),
+      meta: meta
+    }
+  end
+
+  def make_version_struct(%{event: "update"}, changeset, meta) do
+    %Version{
+      event: "create",
+      item_type: changeset.data.__struct__ |> Module.split |> List.last,
+      item_id: changeset.data.id,
+      item_changes: changeset.changes,
+      meta: meta
+    }
+  end
+
+  def make_version_struct(%{event: "destroy"}, model, meta) do
+    %Version{
+      event: "destroy",
+      item_type: model.__struct__ |> Module.split |> List.last,
+      item_id: model.id,
+      item_changes: Map.drop(model, [:__struct__, :__meta__]),
+      meta: meta
+    }
+  end
 end

lib/version.ex

@@ -0,0 +1,33 @@
+defmodule Model.Version do
+  use Ecto.Schema
+
+  import Ecto
+  import Ecto.Changeset
+  import Ecto.Query
+  # how to record column changes in migration
+
+  schema "versions" do
+    field :event, :string
+    field :item_type, :string
+    field :item_id, :integer
+    field :item_changes, :map
+    field :meta, :map
+    field :originator, :string
+
+    timestamps(updated_at: false)
+  end
+
+  @required_fields ~w(item_type item_id event created_at)
+  @optional_fields ~w(meta originator)
+
+  @doc """
+  Creates a changeset based on the `model` and `params`.
+
+  If no params are provided, an invalid changeset is returned
+  with no validation performed.
+  """
+  def changeset(model, params \\ :empty) do
+    model
+    |> cast(params, @required_fields, @optional_fields)
+  end
+end

priv/repo/migrations/20160619190936_add_versions.exs

@@ -0,0 +1,16 @@
+defmodule Repo.Migrations.AddVersions do
+  use Ecto.Migration
+
+  def change do
+    create table(:versions) do
+      add :event,        :string
+      add :item_type,    :string
+      add :item_id,      :integer
+      add :item_changes, :map
+      add :meta,         :map
+      add :originator,   :string
+
+      add :inserted_at,  :datetime, null: false
+    end
+  end
+end