persistence.js

persistence.js is a simple asynchronous Javascript object-relational mapper library. It works with the in-browser HTML5 SQLite database as well as Google Gears' local data store. It may be used to develop offline-capable web applications.

It has no dependencies on any other frameworks, other than the Google Gears initialization script, in case you want to enable Gears support.

What's all this asynchronous stuff about?

In browsers, Javascript and the web page's render engine have to share a single thread. The result of this is that only one thing can happen at a time. If a database query would be performed synchronously, like in many other programming environments, like Java and PHP, the browser would freeze from the moment the query was issued until the results came back, which is clearly suboptimal. Therefore, many APIs in Javascript are defined as asynchronous APIs, which mean that they do not block when an "expensive" computation is performed, but instead provide the call with a function that will be invoked once the result is known. In the meantime, the browser can perform other duties.

For instance, a synchronous database call call would look as follows:

var results = db.query("SELECT * FROM Table");
for(...) { ... }

The execution of the first statement could take half a second, during which the browser doesn't do anything else. By contrast, the asynchronous version looks as follows:

db.query("SELECT * FROM Table", function(results) {
  for(...) { ... }
});

Note that there will be a delay between the db.query call and the result being available and that while the database is processing the query, the execution of the Javascript continues. To make this clear, consider the following program:

db.query("SELECT * FROM Table", function(results) {
  console.log("hello");
});
console.log("world");

Although one could assume this would print "hello", followed by "world", the result will likely be that "world" is printed before "hello", because "hello" is printed when the results from the query are available. This is a tricky thing about asynchronous programming that a Javascript developer will have to get used to.

Browser support

• Modern webkit browsers (Google Chrome and Safari)
• Firefox (through Google Gears)
• Android browser (tested on 1.6 and 2.1)
• iPhone browser (iPhone OS 3+)
• Palm WebOS (tested on 1.4.0)

Internet Explorer is likely not supported (untested) because it lacks __defineGetter__ and __defineSetter__ support, which persistence.js uses heavily. This may change in IE 8.

Connecting to a database

Currently there is one global database connection, which is initialized with a persistence.connect call. Its first argument is the database name, the second a database description and third the maximum database size (in bytes):

persistence.connect('testdb', 'My test db', 5 * 1024 * 1024);

Schema definition

A data model is declared using persistence.define. The following two definitions define a Task and Category entity with a few simple properties. The property types are SQLite types.

var Task = persistence.define('Task', {
  name: "TEXT",
  description: "TEXT",
  done: "BOOL"
});

var Category = persistence.define('Category', {
  name: "TEXT"
});

var Tag = persistence.define('Task', {
  name: "TEXT"
});

The returned values are constructor functions and can be used to create new instances of these entities later:

Relationships between entities are defined using the constructor function's hasMany call:

// This defines a one-to-many relationship:
Category.hasMany('tasks', Task, 'category');
// These two definitions define a many-to-many relationship
Task.hasMany('tags', Tag, 'tasks');
Tag.hasMany('tasks', Task, 'tags');

The first statement defines a tasks relationship on category objects containing a QueryCollection (see the section on query collections later) of Tasks, it also defines an inverse relationship on Task objects with the name category. The last two statements define a many-to-many relationships between Task and Tag. Task gets a tags property (a QueryCollection) containing all its tags and vice versa, Tag gets a tasks property containing all of its tasks.

The defined entity definitions are synchronized (activated) with the database using a persistence.schemaSync call, which takes a callback function (with a newly created transaction as an argument), that is called when the schema synchronization has completed, the callback is optional.

persistence.schemaSync();
// or
persistence.schemaSync(function(tx) { 
  // tx is the transaction object of the transaction that was
  // automatically started
});

Creating and manipulating objects

New objects can be instantiated with the constructor functions. Optionally, an object with initial property values can be passed as well, or the properties may be set later:

var task = new Task();
var category = new Category({name: "My category"});
var tag = new Tag();
tag.name = "work";

Many-to-one relationships are accessed using their specified name, e.g.: task.category = category;

One-to-many and many-to-many relationships are access and manipulated through the QueryCollection API that will be discussed later:

task.tags.add(tag);
tasks.tags.remove(tag)l
tasks.tags.list(tx, function(allTags) { console.log(allTags); });

Persisting/removing objects

Similar to hibernate, persistence.js uses a tracking mechanism to determine which objects' changes have to be persisted to the datase. All objects retrieved from the database are automatically tracked for changes. New entities can be tracked to be persisted using the persistence.add function:

var c = new Category({name: "Main category"});
persistence.add(c);
for ( var i = 0; i < 5; i++) {
  var t = new Task();
  t.name = 'Task ' + i;
  t.done = i % 2 == 0;
  t.category = c;
  persistence.add(t);
}

Objects can also be removed from the database:

persistence.remove(c);

All changes made to tracked objects can be flushed to the database by using persistence.flush, which takes a transaction object and callback function as arguments. A new transaction can be started using persistence.transaction:

persistence.transaction(function(tx) {
  persistence.flush(tx, function() {
    alert('Done flushing!');
  });
});

For convenience, it is also possible to not specify a transaction or callback, in that case a new transaction will be started automatically. For instance:

persistence.flush();
// or, with callback
persistence.flush(null, function() {
  alert('Done flushing');
});

Note that when no callback is defined, the flushing still happens asynchronously.

Important: Changes and new objects will not be persisted until you explicitly call persistence.flush(). The exception to this rule is using the list(...) method on a database QueryCollection, which also flushes first, although this behavior may change in the future.

Dumping (and restoring) a database

persistence.dump can be used to create an object containing a full dump of a database. Naturally, it is adviced to only do this with smaller databases. Example:

persistence.dump(tx, [Task, Category], function(dump) {
  console.log(dump);
});

When null is provided as a first argument a new transaction will be started for the operation. If null is provided as second argument, dump defaults to dumping all defined entities.

The dump format is:

{"entity-name": [list of instances],
 ...}

There is no restore API yet, but this will be implemented soon.

Query collections

A core concept of persistence.js is the QueryCollection. A QueryCollection represents a (sometimes) virtual collection that can be filtered, ordered or paginated. QueryCollections are somewhate inspired by Google AppEngine's Query class. A QueryCollection has the following methods:

• filter(property, operator, value)
  Returns a new QueryCollection that adds a filter, filtering a certain property based on an operator and value. Supported operators are '=', '!=', '<', '<=', '>' and '>='. Example: .filter('done', '=', true)
• order(property, ascending)
  Returns a new QueryCollection that will order its results by the property specified in either an ascending (ascending === true) or descending (ascending === false) order.
• limit(n)
  Returns a new QueryCollection that limits the size of the result set to n items. Useful for pagination.
• skip(n)
  Returns a new QueryCollection that skips the first n results. Useful for pagination.
• prefetch(rel)
  Returns a new QueryCollection that prefetches entities linked through relationship rel, note that this only works for one-to-one and many-to-one relationships.
• list(tx, callback)
  Asynchronously fetches the results matching the formulated query. Once retrieved, the callback function is invoked with an array of entity objects as argument.
• each(tx, eachCallback)
  Asynchronously fetches the results matching the formulated query. Once retrieved, the eachCallback function is invoked on each element of the result objects.
• add(obj)
  Adds object obj to the collection.
• remove(obj)
  Removes object obj from the collection.

Query collections are returned by:

• EntityName.all(), e.g. Task.all()
• one-to-many and many-to-many relationships, e.g. task.tags

Example:

var allTasks = Task.all().filter("done", '=', true).prefetch("category").order("name", false).limit(10);
    
allTasks.list(null, function (results) {
    results.forEach(function (r) {
        console.log(r.name)
        window.task = r;
    });
});

Limitations/Plans

persistence.js is still in its early development stages and not extensively tested so there may be many bugs. If you find a bug, please report it. or fork the project, fix the problem and send me a pull request. For a list of planned features and open issues, have a look at the issue tracker.

License

This work is licensed under the MIT license.