Yariv’s Blog

The title says it all :)

Not only that, I also added more powerful SELECT query capabilities to many-to-one relations.

Here’s how it works:

Add these tables to erlydb.sql


create table developer(
        id integer auto_increment primary key,
        name varchar(30),
        country varchar(20));
 
create table developer_project(
        developer_id integer,
        project_id integer,
        primary key(developer_id, project_id));


This is our modified project.erl file:


-module(project).
-compile(export_all).
 
relations() ->
    [{many_to_one, [language]},
     {many_to_many, [developer]}].


This is our new developer.erl files:


-module(developer).
-export([relations/0]).
 
relations() ->
    [{many_to_many, [project]}].


Following is some code demonstrating the new capabilities. The full module is in the distribution, in test/erlydb_test.erl.


    %% fancier project queries
    {ok, [Yaws4]} = language:projects_where(
          Erlang, "name='Yaws'"),
    true = Yaws4 == Yaws1,
 
    {ok, [Yaws5]} = language:projects_with(
          Erlang, "limit 1"),
    true = Yaws4 == Yaws5,
 
    {ok, [Ejabberd4]} = language:projects(
        Erlang, "name like '%e%'", "limit 1"),
    true = Ejabberd4 == Ejabberd3,
 
    %% Let's show off some many-to-many features
 
    %% First, add some more projects
    [OTP, Mnesia] =
  lists:map(
    fun(Proj) ->
      {ok, P1} = project:save(Proj),
      P1
    end,
    [project:new("OTP", "The Open Telephony Platform", Erlang),
     project:new("Mnesia", "A distributed database " ++
           "engine written in Erlang", Erlang)]),
 
    %% Next, add some developers
    [Joe, Ulf, Klacke] =
  lists:map(
    fun(Developer) ->
      {ok, D1} =
          developer:save(Developer),
      D1
    end,
    [developer:new("Joe Armstrong", "Sweden"),
     developer:new("Ulf Wiger", "Sweden"),
     developer:new("Claes (Klacke) Wikstrom", "Sweden")]),
 
    %% Add some developers to our projects
    ok = project:add_developer(OTP, Joe),
    ok = project:add_developer(OTP, Klacke),
    ok = project:add_developer(OTP, Ulf),
    
    %% Add some projects to our developers
    ok = developer:add_project(Klacke, Yaws1),
    ok = developer:add_project(Klacke, Mnesia),
    ok = developer:add_project(Ulf, Mnesia),
 
    
    %% basic SELECT query
    {ok, [Joe, Ulf, Klacke]} =
  project:developers(OTP),
 
    %% fancier SELECT queries
    {ok, [Ulf, Klacke]} =
  project:developers(Mnesia),
    {ok, [Ulf]} = project:developers_with(Mnesia,
            "limit 1"),
    {ok, [Klacke]} =
  project:developers_where(Mnesia,
         "name like 'Claes%'"),
    
    %% SELECT query, from the other direction
    {ok, [Yaws1, OTP, Mnesia]} =
  developer:projects(Klacke),
 
    %% Klacke, nothing personal here :)
    ok = developer:remove_project(Klacke, Yaws1),
    {ok, [OTP, Mnesia]} = developer:projects(Klacke),
    ok = developer:remove_project(Klacke, OTP),
    {ok, [Mnesia]} = developer:projects(Klacke),
    ok = developer:remove_project(Klacke, Mnesia),
    {ok, []} = developer:projects(Klacke),
 
    %% I forgot to show you can delete records, too :)
    ok = language:delete(Java).


ErlyDB is still in alpha version, so you may have problems. Please let me know if you find any bugs :)

(FYI, I’m moving to Boston tomorrow, so I will probably not be of much help in the next few days.)

Enjoy!