# Musiksammlung (JavaScript)

Erstellung einer kleinen Datenbankanwendung zum Verwalten Ihrer Musiksammlung
GNOME-Dokumentationsprojekt
gnome-doc-list@gnome.org
Johannes Schmid
jhs@gnome.org
Marta Maria Casetti
mmcasettii@gmail.com 2013
Mario Blättermann
mario.blaettermann@gmail.com 2011, 2013, 2016, 2018

# Musiksammlung

      In diesem Tutorial lernen Sie, wie:

        Wie eine Verbindung zu einer Datenbank mittels libgda erstellt wird
        Datensätze in eine Datenbanktabelle eingefügt werden oder wie die Tabelle durchsucht werden kann

# Einführung

This demo uses the Javascript language. We are going to demonstrate how to connect and use a database from a GTK program, by using the GDA (GNOME Data Access) library. Thus you also need this library installed.

GNOME Data Access (GDA) is library whose purpose is to provide universal access to different kinds and types of data sources. This goes from traditional relational database systems, to any imaginable kind of data source such as a mail server, a LDAP directory, etc. For more information, and for a full API and documentation, visit the GDA website.

Although a big part of the code is related to user interface (GUI), we are going to focus our tutorial on the database parts (we might mention other parts we think are relevant though). To know more about Javascript programs in GNOME, see the Image Viewer program tutorial.

# Erstellen eines Projekts in Anjuta

Before you start coding, you'll need to set up a new project in Anjuta. This will create all of the files you need to build and run the code later on. It's also useful for keeping everything together.

Start Anjuta and click FileNewProject to open the project wizard.

Choose Generic Javascript from the JS tab, click Forward, and fill-out your details on the next few pages. Use record-collection as project name and directory.

Click Finished and the project will be created for you. Open src/main.js from the Project or File tabs. It contains very basic example code.

# Programmstruktur

This demo is a simple GTK application (with a single window) capable of inserting records into a database table as well as browsing all records of the table. The table has two fields: `id`, an integer, and `name`, a varchar. The first section (on the top) of the application allows you to insert a record into the table. The last section (bottom) allows you to see all the records of that table. Its content is refreshed every time a new record is inserted and on the application startup.

# Starting the fun

Let's start by examining the skeleton of the program:

const GLib = imports.gi.GLib; const Gtk = imports.gi.Gtk; const Gda = imports.gi.Gda; const Lang = imports.lang;

function Demo () { this._init (); }

Demo.prototype = {

_init: function () { this.setupWindow (); this.setupDatabase (); this.selectData (); } }

Gtk.init (null, null);

var demo = new Demo ();

Gtk.main ();

Lines 1‒4: Initial imports. Pay special attention to line 3, which tells Javascript to import the GDA library, our focus in this tutorial.

Lines 6‒17: Define our `Demo` class. Pay special attention to lines 13‒15, where we call 3 methods which will do the whole job. They will be detailed below.

Lines 19‒23: Start the application.

# Entwurf der Anwendung

Let's take a look at the `setupWindow` method. It is responsible for creating the User Interface (UI). As UI is not our focus, we will explain only the relevant parts.

setupWindow: function () { this.window = new Gtk.Window ({title: "Data Access Demo", height_request: 350}); this.window.connect ("delete-event", function () { Gtk.main_quit(); return true; });

// main box var main_box = new Gtk.Box ({orientation: Gtk.Orientation.VERTICAL, spacing: 5}); this.window.add (main_box);

// first label var info1 = new Gtk.Label ({label: "<b>Insert a record</b>", xalign: 0, use_markup: true}); main_box.pack_start (info1, false, false, 5);

// "insert a record" horizontal box var insert_box = new Gtk.Box ({orientation: Gtk.Orientation.HORIZONTAL, spacing: 5}); main_box.pack_start (insert_box, false, false, 5);

// ID field insert_box.pack_start (new Gtk.Label ({label: "ID:"}), false, false, 5); this.id_entry = new Gtk.Entry (); insert_box.pack_start (this.id_entry, false, false, 5);

// Name field insert_box.pack_start (new Gtk.Label ({label: "Name:"}), false, false, 5); this.name_entry = new Gtk.Entry ({activates_default: true}); insert_box.pack_start (this.name_entry, true, true, 5);

// Insert button var insert_button = new Gtk.Button ({label: "Insert", can_default: true}); insert_button.connect ("clicked", Lang.bind (this, this._insertClicked)); insert_box.pack_start (insert_button, false, false, 5); insert_button.grab_default ();

// Browse textview var info2 = new Gtk.Label ({label: "<b>Browse the table</b>", xalign: 0, use_markup: true}); main_box.pack_start (info2, false, false, 5); this.text = new Gtk.TextView ({editable: false}); var sw = new Gtk.ScrolledWindow ({shadow_type:Gtk.ShadowType.IN}); sw.add (this.text); main_box.pack_start (sw, true, true, 5);

this.count_label = new Gtk.Label ({label: "", xalign: 0, use_markup: true}); main_box.pack_start (this.count_label, false, false, 0);

this.window.show_all (); },

Lines 22 and 27: Create the 2 entries (for the two fields) in which users will type something to get inserted in the database.

Lines 31‒34: Create the Insert button. We connect its `clicked` signal to the `_insertClicked` private method of the class. This method is detailed below.

Line 39: Create the widget (`TextView`) where we will show the contents of the table.

Line 44: Create the label where we will show the number of records in the table. Initially it's empty, it will be updated later.

# Connecting to and initializing the database

The code which makes the connection to the database is in the `setupDatabase` method below:

setupDatabase: function () { this.connection = new Gda.Connection ({provider: Gda.Config.get_provider("SQLite"), cnc_string:"DB_DIR=" + GLib.get_home_dir () + ";DB_NAME=gnome_demo"}); this.connection.open ();

try { var dm = this.connection.execute_select_command ("select * from demo"); } catch (e) { this.connection.execute_non_select_command ("create table demo (id integer, name varchar(100))"); } },

Lines 2‒3: Create the GDA's `Connection` object. We must supply to its constructor some properties:

`provider`: One of GDA's supported providers. GDA supports SQLite, MySQL, PostgreSQL, Oracle and many others. For demo purposes we will use a SQLite database, as it comes installed by default in most distributions and it is simple to use (it just uses a file as a database).

`cnc_string`: The connection string. It may change from provider to provider. The syntax for SQLite is: `DB_DIR=PATH;DB_NAME=FILENAME`. In this demo we are accessing a database called gnome_demo in the user home dir (note the call to GLib's `get_home_dir` function).

If the provider is not supported by GDA, or if the connection string is missing some element, line 2 will raise an exception. So, in real life we should handle it with JavaScript's statement `try`...`catch`.

Line 4: Open the connection. In the SQLite provider, if the database does not exist, it will be created in this step.

Lines 6‒10: Try to do a simple select to check if the table exists (line 7). If it does not exist (because the database was just created), this command will raise an exception, which is handled by the `try`...`catch` block. If it is the case, we run the create table statement (line 9).

In order to run the SQL commands above we are using the GDA connection methods `execute_select_command` and `execute_non_select_command`. They are simple to use, and just require two arguments: The `Connection` object and the SQL command to be parsed.

At this point we have the database set up, and are ready to use it.

# Auswählen

After connecting to the database, our demo's constructor calls the `selectData` method. It is responsible for getting all the records in the table and showing them on the `TextView` widget. Let's take a look at it:

selectData: function () { var dm = this.connection.execute_select_command ("select * from demo order by 1, 2"); var iter = dm.create_iter ();

var text = "";

while (iter.move_next ()) { var id_field = Gda.value_stringify (iter.get_value_at (0)); var name_field = Gda.value_stringify (iter.get_value_at (1));

text += id_field + "\t=>\t" + name_field + '\n'; }

this.text.buffer.text = text; this.count_label.label = "<i>" + dm.get_n_rows () + " record(s)</i>"; },

Line 2: The `SELECT` command. We are using the GDA connection's `execute_select_command` method for that. It returns a `DataModel` object, which is later used to retrieve the rows.

Line 3: Create an `Iter` object, which is used to iterate over the `DataModel`'s records.

Line 7: Loop through all the records, fetching them with the help of the `Iter` object. At this point, the `iter` variable contains the actual, retrieved data. Its `move_next` method returns `false` when it reaches the last record.

Lines 8‒9: We do two things in each line:

Use `Iter`'s method `get_value_at`, which requires only one argument: the column number to retrieve, starting at 0. As our `SELECT` command returns only two columns, we are retrieving columns 0 and 1.

The method `get_value_at` returns the field in GLib's `GValue` format. A simple way to convert this format to a string is by using GDA's global function `value_stringify`. That's what we are doing here, and we store the results in the variables `id_field` and `name_field`.

Line 11: Concatenate the two fields to make one text line, separated by `"=>"`, and store it in the `text` variable.

Line 14: After the loop is finished, we have all the records formatted in the `text` variable. In this line we just set the contents of the `TextView` with that variable.

Line 15: Display the number of records in the table, making use of the `DataModel`'s `get_n_rows` method.

# Einfügen

OK, we know how to connect to a database and how to select rows from a table. Now it's time to do an `INSERT` on the table. Do you remember above, in the method `setupWindow` we connected the Insert button's `clicked` signal to the method `_insertClicked`? Let's see the implementation of this method.

_insertClicked: function () { if (!this._validateFields ()) return;

// Gda.execute_non_select_command (this.connection, // "insert into demo values ('" + this.id_entry.text + "', '" + this.name_entry.text + "')");

var b = new Gda.SqlBuilder ({stmt_type:Gda.SqlStatementType.INSERT}); b.set_table ("demo"); b.add_field_value_as_gvalue ("id", this.id_entry.text); b.add_field_value_as_gvalue ("name", this.name_entry.text); var stmt = b.get_statement (); this.connection.statement_execute_non_select (stmt, null);

this._clearFields (); this.selectData (); },

We have learned how to use the GDA connection's methods `execute_select_command` and `execute_non_select_command` to quickly execute SQL commands on the database. GDA allows one to build a SQL statement indirectly, by using its `SqlBuilder` object. What are the benefits of this? GDA will generate the SQL statement dynamically, and it will be valid for the connection provider used (it will use the same SQL dialect the provider uses). Let's study the code:

Lines 2‒3: Check if the user filled all the fields. The code for the private method `_validateFields` is really simple and you can read it in the full demo source code.

Line 5: The faster way of doing the `INSERT`. It's commented out as we want to show how to use the `SqlBuilder` object to build a SQL statement portable across databases.

Line 7: Create the `SqlBuilder` object. We must pass the type of statement we are going to build. It can be `SELECT`, `UPDATE`, `INSERT` or `DELETE`.

Line 8: Set the name of the table on which the built statement will operate (it will generate `INSERT INTO demo`)

Lines 9‒10: Set the fields and its values that will be part of the statement. The first argument is the field name (as in the table). The second one is the value for that field.

Line 11: Get the dynamically generated `Statement` object, which represents a SQL statement.

Line 12: Finally, execute the SQL statement (`INSERT`).

Line 14: Clear the id and name fields on the screen. The code for the private method `_clearFields` is really simple and you can read it in the full demo source code.

Line 15: Refresh the view on the screen by doing another `SELECT`.

You can also make use of parameters while building the statement. By using the `SqlBuilder` objects and parameters you are less subject to attacks like SQL injection. Check the GDA documentation for more information about parameters.

# Anwendung ausführen

All of the code you need should now be in place, so try running the code. You now have a database for your record collection!

# Referenz-Implementierung

If you run into problems with the tutorial, compare your code with this reference code.