2 posts tagged with "erl"

In the last post we got epgsql running inside our Erlang REPL.  Now we are going to start using epgsql.

Start your Postgres server and open up your REPL.  Make sure epgsql is loaded by running:

m(epgsql).

If it's there you will see the module's information:

To connect to our database we run:

{ok, Connection} = epgsql:connect("<hostname>", "<username>", "<password>", [{database, "<dbname>"}]).

This will return a connection that we can then use to interact with the database.

To create a table by running the following Simple Query:

{ok, [], []} = epgsql:squery(Connection, "CREATE TABLE foo (id SERIAL PRIMARY KEY, prefix TEXT, suffix TEXT);").

Now to insert data into our database we run:

{ok, 1} = epgsql:squery(Connection, "INSERT INTO foo (prefix, suffix) VALUES ('foo', 'bar');").

This returns {ok, N}, where N is the number of rows inserted.  Lets go head and add two more items into out database:

{ok, 2} = epgsql:squery(Connection, "INSERT INTO foo  (prefix, suffix) VALUES ('one', 'two'), ('three', 'four');").

In order to query our database we can use a simple query

{ok, Cols, Rows} = epgsql:squery(Connection, "SELECT * FROM foo;").

This will return all the data in the row as binary data:

In order to get data returned typed correctly we need to use an extended query:

{ok, Cols, Rows} = epgsql:equery(Connection, "SELECT * FROM foo;", []).

As you can see, the id column is an integer now, the strings are still binary.  However, if we had booleans they would be returned as boolean values, etc.

That's how you can connect to and get data in and out of a Postgres database using Erlang.

Now lets close the connection by running:

epgsql:close(Connection).

I've been trying to get epgsql so I can build out the database for Markov-lang.  Getting this running took some time.

First we needed to install rebar3.  You do this by running:

brew install homebrew/versions/rebar3

Once rebar3 is installed, you have to configure it to work with hex.pm.  You do this by running the following:

mkdir -p ~/.config/rebar3 

Then create the following file ~/.config/rebar3/rebar.config with the following contents:

 {plugins, [rebar3_hex]}.

(EDIT: According to Reddit user mononcqc this is only necessary if you want to publish packages not fetch them)

Then run:

rebar3 update

Now, in the directory for you project that will use postgres, create a file /Path/To/App/rebar.config with the following content:

{deps, [epgsql]}.

Now you can access epgsql by running:

rebar3 shell

It should compoile epgsql. In the REPL started by the previous command you can run:

m(epgsql).

To access this from within Emacs, start the Erlang REPL in Emacs with the following flag:

-pz _build/default/deps/epgsql/ebin 

Part 2 can be found here.