defmodule JsonApiQueryBuilder do
  @moduledoc """
  Behaviour and mixin for building an Ecto query from a JSON-API request.

  ## Example

      defmodule Article do
        use Ecto.Schema

        schema "articles" do
          field :body, :string
          field :description, :string
          field :slug, :string
          field :tag_list, {:array, :string}
          field :title, :string
          belongs_to :author, User, foreign_key: :user_id
          has_many :comments, Comment
          timestamps()
        end

        defmodule Query do
          use JsonApiQueryBuilder,
            schema: Article,
            type: "article",
            relationships: ["author", "comments"]

          @impl JsonApiQueryBuilder
          def filter(query, "tag", value), do: from(a in query, where: ^value in a.tag_list)
          def filter(query, "comments", params) do
            comment_query = from(Comment.Query.build(params), select: [:article_id], distinct: true)
            from a in query, join: c in ^subquery(comment_query), on: a.id == c.article_id
          end
          def filter(query, "author", params) do
            user_query = from(User.Query.build(params), select: [:id])
            from a in query, join: u in ^subquery(user_query), on: a.user_id == u.id
          end

          @impl JsonApiQueryBuilder
          def include(query, "comments", comment_params) do
            from query, preload: [comments: ^Comment.Query.build(comment_params)]
          end
          def include(query, "author", author_params) do
            from query, preload: [author: ^User.Query.build(author_params)]
          end
        end
      end
  """

  @typedoc """
  A JSON-API request after parsing by Plug into a string keyed map.

  May contain `"filter"`, `"sort"`, `"fields"`, `"include"`, `"page"` keys.
  """
  @type request :: %{String.t => any}

  @doc """
  Builds an `Ecto.Queryable.t` from parsed JSON-API request parameters.

  An overridable default implementation is generated by the mixin.

  ## Example:

      User.Query.build(%{
        "filter" => %{
          "articles.tag" => "animals",
          "comments" => %{
            "body" => "Boo"
          }
        },
        "include" => "articles.comments",
        "fields" => %{"user" => "id,bio"}
      })

      #Ecto.Query<
        from u in Blog.User,
        join: a in ^#Ecto.Query<
          from a in subquery(
            from a in Blog.Article,
            where: ^"animals" in a.tag_list,
            distinct: true,
            select: [:user_id]
          )
        >,
        on: u.id == a.user_id,
        select: [:id, :bio],
        preload: [
          articles: #Ecto.Query<
            from a in Blog.Article,
            preload: [comments: #Ecto.Query<from c in Blog.Comment>]
          >
        ]
      >
  """
  @callback build(request) :: Ecto.Queryable.t

  @doc """
  Applies filter conditions from a parsed JSON-API request to an `Ecto.Queryable.t`

  An overridable default implementation is generated by the mixin.
  """
  @callback filter(query :: Ecto.Queryable.t, request) :: Ecto.Queryable.t


  @doc """
  Callback responsible for adding a filter criteria to a query.

  Attribute filters will generally add a `where:` condition to the query.

  Relationship filters will generally add a `join:` based on a subquery.

  When applying a filter to a has-many relationship, take care to `select:` the foreign key with `distinct: true` to avoid duplicated results.
  For filtering a belongs-to relationships, selecting the primary key is all that is needed.

  ## Example

      @impl JsonApiQueryBuilder
      def filter(query, "tag", value), do: from(article in query, where: ^value in article.tag_list)
      def filter(query, "comments", params) do
        comment_query = from(Comment.Query.build(params), select: [:article_id], distinct: true)
        from article in query, join: comment in ^subquery(comment_query), on: article.id == comment.article_id
      end
      def filter(query, "author", params) do
        user_query = from(User.Query.build(params), select: [:id])
        from article in query, join: user in ^subquery(user_query), on: article.user_id == user.id
      end
  """
  @callback filter(query :: Ecto.Queryable.t, field :: String.t, value :: any) :: Ecto.Queryable.t


  @doc """
  Applies sparse fieldset selection from a parsed JSON-API request to an `Ecto.Queryable.t`

  An overridable default implementation is generated by the mixin.
  """
  @callback fields(query :: Ecto.Queryable.t, request) :: Ecto.Queryable.t

  @doc """
  Optional callback responsible for mapping a JSON-API field string to an Ecto schema field.

  An overridable default implementation using `String.to_existing_atom/1` is generated by the mixin.

  ## Example

      @impl JsonApiQueryBuilder
      def field("username"), do: :name
      def field("price"), do: :unit_price
      def field(other), do: String.to_existing_atom(other)
  """
  @callback field(api_field :: String.t) :: atom

  @doc """
  Applies sorting from a parsed JSON-API request to an `Ecto.Queryable.t`

  An overridable default implementation is generated by the mixin.
  """
  @callback sort(query :: Ecto.Queryable.t, request) :: Ecto.Queryable.t

  @doc """
  Applies related resource inclusion from a parsed JSON-API request to an `Ecto.Queryable.t` as preloads.

  An overridable default implementation is generated by the mixin.
  """
  @callback include(query :: Ecto.Queryable.t, request) :: Ecto.Queryable.t

  @doc """
  Callback responsible for adding an included resource via `preload`.

  ## Example

      @impl JsonApiQueryBuilder
      def include(query, "comments", comment_params) do
        from query, preload: [comments: ^Comment.Query.build(comment_params)]
      end
      def include(query, "author", author_params) do
        from query, preload: [author: ^User.Query.build(author_params)]
      end
  """
  @callback include(query :: Ecto.Queryable.t, relationship :: String.t, related_request :: request) :: Ecto.Queryable.t


  @doc false
  defmacro __using__(schema: schema, type: type, relationships: relationships) do
    quote do
      import Ecto.Query

      @behaviour JsonApiQueryBuilder

      @schema unquote(schema)
      @api_type unquote(type)
      @relationships unquote(relationships)

      @impl JsonApiQueryBuilder
      def build(params) do
        @schema
        |> from()
        |> filter(params)
        |> fields(params)
        |> sort(params)
        |> include(params)
      end

      @impl JsonApiQueryBuilder
      def filter(query, params) do
        JsonApiQueryBuilder.Filter.filter(query, params, &filter/3, relationships: @relationships)
      end

      @impl JsonApiQueryBuilder
      def fields(query, params) do
        JsonApiQueryBuilder.Fields.fields(query, params, &field/1, type: @api_type)
      end

      @impl JsonApiQueryBuilder
      def sort(query, params) do
        JsonApiQueryBuilder.Sort.sort(query, params, &field/1)
      end

      @impl JsonApiQueryBuilder
      def include(query, params) do
        JsonApiQueryBuilder.Include.include(query, params, &include/3)
      end

      @impl JsonApiQueryBuilder
      def field(str), do: String.to_existing_atom(str)

      defoverridable [build: 1, filter: 2, fields: 2, field: 1, sort: 2, include: 2]
    end
  end
end