Lethe

Lethe /ˈli θi/ • noun ^{(try it with IPA Reader)}

1. Classical Mythology. A river in Hades whose water caused forgetfulness of the past in those who drank of it.

A marginally-better (WIP) query DSL for Mnesia. Originally implemented for 신경.

Matchspecs suck, so this is a sorta-better alternative.

WARNING: Currently, only read operations are supported. This may or may not change in the future.

Things that may trip you up

• The map functions is_map_key and map_get take arguments in the order (key, map), NOT (map, key)!

Roadmap

• Select all fields of a record
• Select some fields of a record
• Limit number of records returned
• Query operators
  • +, -, *, div, rem, >, >=, <, <=, !=
  • Bitwise operators
  • Tuple, list, and map operators (map_size, hd, tl, element, etc.)
  • Misc. math functions (abs, trunc, etc.)
  • Boolean operators
    • Logical AND/OR/etc.
    • is_pid/is_binary/etc.
• Write DSL

Installation

Get it on Hex.

Read the docs.

Usage

# Create a table...
table = :table
:mnesia.create_schema []
:mnesia.start()
:mnesia.create_table table, [attributes: [:integer, :string, :map]]

# ...and add some indexes...
:mnesia.add_table_index table, :integer
:mnesia.add_table_index table, :string
:mnesia.add_table_index table, :map

# ...and some test data.
n = fn -> :rand.uniform 1_000_000_000_000_000_000_000_000_000_000_000_000_000_000_000 end
for i <- 1..10_000, do: :mnesia.dirty_write {table, i, "#{n.()}", %{i => "#{n.()}"}}

# Now let's run some queries!
# Lethe's query DSL allows you to use the names of your table attributes,
# rather than forcing you to think about what their index in the record is, or
# anything else like that.
# Currently, Lethe requires that you compile your queries before running them.
# This is primarily done to aid in debugging, and a `Lethe.compile_and_run/1`
# is likely to happen in the future.

# Select one record and all its fields
{:ok, [{integer, string, map}]} =
  table
  |> Lethe.new
  |> Lethe.limit(1)
  |> Lethe.select_all
  |> Lethe.compile
  |> Lethe.run

# Select all fields from all records
# `Lethe.select_all` and `Lethe.limit(:all)` are the default settings.
{:ok, all_records} =
  table
  |> Lethe.new
  |> Lethe.compile
  |> Lethe.run

# Select a single field from a single record
{:ok, [integer]} =
  table
  |> Lethe.new
  |> Lethe.limit(1)
  |> Lethe.select(:integer)
  |> Lethe.compile
  |> Lethe.run

# Select a bunch of records at once
{:ok, records} =
  table
  |> Lethe.new
  |> Lethe.limit(100)
  |> Lethe.compile
  |> Lethe.run

# Select specific fields from a record
{:ok, [{int, map}, {int, map}]} =
  table
  |> Lethe.new
  |> Lethe.limit(2)
  |> Lethe.select([:integer, :map])
  |> Lethe.compile
  |> Lethe.run

# Now let's use some operators!
# Lethe internally rewrites all of these expressions into Mnesia guard form.

# Select all values where :integer * 2 <= 10
{:ok, res} =
  table
  |> Lethe.new
  |> Lethe.select(:integer)
  |> Lethe.where(:integer * 2 <= 10)
  |> Lethe.compile
  |> Lethe.run

# Select all values where :integer * 2 >= 4 and :integer * 2 <= 10
{:ok, res} =
  table
  |> Lethe.new
  |> Lethe.select(:integer)
  |> Lethe.where(:integer * 2 >= 4 and :integer * 2 <= 10)
  |> Lethe.compile
  |> Lethe.run

# An example of a very complicated query
{:ok, res} =
  table
  |> Lethe.new
  |> Lethe.select(:integer)
  |> Lethe.where(
    :integer * 2 == 666
      and is_map(:map)
      and is_map_key(:integer, :map)
      and map_get(:integer, :map) == :string
  )
  |> Lethe.compile
  |> Lethe.run

# Using external variables in queries
i = 333

{:ok, res} =
  table
  |> Lethe.new
  |> Lethe.select(:integer)
  |> Lethe.where(:integer * 2 == ^i * 2)
  |> Lethe.compile
  |> Lethe.run

# Using atom literals in queries
{:ok, res} =
  table
  |> Lethe.new
  |> Lethe.select(:atom)
  |> Lethe.where(:atom == &:atom)
  |> Lethe.compile
  |> Lethe.run

# See the documentation on `Lethe.where/2` for a list of all available ops