# needed to get defdatabase and other macros
use Amnesia

# defines a database called Database, it's basically a defmodule with
# some additional magic
defdatabase Database do
  # this is just a forward declaration of the table, otherwise you'd have
  # to fully scope User.read in Message functions
  deftable User

  # this defines a table with an user_id key and a content attribute, and
  # makes the table a bag; tables are basically records with a bunch of helpers
  deftable Message, [:user_id, :content], type: :bag do
    # this isn't required, but it's always nice to spec things
    @type t :: %Message{user_id: integer, content: String.t}

    # this defines a helper function to fetch the user from a Message record
    def user(self) do
      User.read(self.user_id)
    end

    # this does the same, but uses dirty operations
    def user!(self) do
      User.read!(self.user_id)
    end
  end

  # this defines a table with other attributes as ordered set, and defines an
  # additional index as email, this improves lookup operations
  deftable User, [{ :id, autoincrement }, :name, :email], type: :ordered_set, index: [:email] do
    # again not needed, but nice to have
    @type t :: %User{id: non_neg_integer, name: String.t, email: String.t}

    # this is a helper function to add a message to the user, using write
    # on the created records makes it write to the mnesia table
    def add_message(self, content) do
      %Message{user_id: self.id, content: content} |> Message.write
    end

    # like above, but again with dirty operations, the bang methods are used
    # thorough amnesia to be the dirty counterparts of the bang-less functions
    def add_message!(self, content) do
      %Message{user_id: self.id, content: content} |> Message.write!
    end

    # this is a helper to fetch all messages for the user
    def messages(self) do
      Message.read(self.id)
    end

    # like above, but with dirty operations
    def messages!(self) do
      Message.read!(self.id)
    end
  end
end

# You want to be in a transaction most of the time, this ensures the data
# doesn't get corrupted and you get meaningful values back.
#
# Most operations won't work outside a transaction and will raise an exception.
Amnesia.transaction do
  # Every table is a record, so you can do everything you can do with records.
  #
  # Once you want to save the record, you have to call `.write` on it, this
  # will write the record to the table.
  #
  # Since we defined the `User` table with an `autoincrement` id attribute it
  # will be incremented internally on write, unless the id attribute is set, in
  # that case it will be left as is.
  #
  # If you want to know the values of the autoincrement fields, `.write` always
  # returns the updated record.
  john = %User{name: "John", email: "john@example.com"} |> User.write

  # Let's create more users.
  richard = %User{name: "Richard", email: "richard@example.com"} |> User.write
  linus   = %User{name: "Linus", email: "linus@example.com"} |> User.write

  # Now let's add some messages.

  john |> User.add_message %S"""
  When we program a computer to make choices intelligently after determining
  its options, examining their consequences, and deciding which is most
  favorable or most moral or whatever, we must program it to take an attitude
  towards its freedom of choice essentially isomorphic to that which a human
  must take to his own.
  """

  john |> User.add_message %S"""
  He who refuses to do arithmetic is doomed to talk nonsense."
  """

  john |> User.add_message %S"""
  It's difficult to be rigorous about whether a machine really 'knows',
  'thinks', etc., because we're hard put to define these things. We understand
  human mental processes only slightly better than a fish understands swimming.
  """

  richard |> User.add_message %S"""
  For personal reasons, I do not browse the web from my computer. (I also have
  no net connection much of the time.) To look at page I send mail to a daemon
  which runs wget and mails the page back to me. It is very efficient use of my
  time, but it is slow in real time.
  """

  richard |> User.add_message %S"""
  I am skeptical of the claim that voluntarily pedophilia harms children. The
  arguments that it causes harm seem to be based on cases which aren't
  voluntary, which are then stretched by parents who are horrified by the idea
  that their little baby is maturing.
  """

  linus |> User.add_message %S"""
  Portability is for people who cannot write new programs.
  """

  linus |> User.add_message %S"""
  Really, I'm not out to destroy Microsoft. That will just be a completely
  unintentional side effect.
  """

  linus |> User.add_message %S"""
  Modern PCs are horrible. ACPI is a complete design disaster in every way. But
  we're kind of stuck with it. If any Intel people are listening to this and
  you had anything to do with ACPI, shoot yourself now, before you reproduce.
  """
end

Amnesia.transaction do
  # The simplest way to read a record is using the key of the record (by
  # default the first attribute)
  #
  # Since we wrote the John, Richard and Linus in this order and the id is
  # defined as *autoincrement*, the first `User` will be John.
  john = User.read(1)

  # Now let's read his messages and print them all.
  john |> User.messages |> Enum.each &IO.puts(&1.content)

  # You can also use an Exquisite selector to fetch records.
  selection = Message.where user_id == 1 or user_id == 2,
    select: content

  # Get the values in the selector and print them.
  selection |> Amnesia.Selection.values |> Enum.each &IO.puts(&1.content)
end