# Apostrophe's model layer: working with the database

Apostrophe provides a model layer (database layer) that gives you convenient ways to read and write docs programmatically, taking advantage of the same features that underpin modules like apostrophe-pieces. Using these features where possible ensures that permissions are respected, widgets are loaded, joins are fetched, versions are recorded for rollback... so many good things.

That being said, you can also access the database directly and there's a time and place for that too, most frequently when you must use $set, $inc, $unset, $push, $pull or $addToSet and you are confident you've already determined the user should be allowed to do something.

# Fetching pieces with Apostrophe

Most of the time, you don't need to work with Apostrophe's collections directly. Modules that subclass apostrophe-pieces have a find() method which returns an Apostrophe cursor you can use to fetch documents with the benefit of support for Apostrophe's joins and other cursor filters. And permissions are taken into account, so that logged-out users can't see what they shouldn't.

For instance, in a subclass of apostrophe-pieces you might write:

return self.find(req, { age: { $gte: 50 }})
  .toArray(function(err, pieces) { ... })

You may also use promises. Apostrophe returns a promise if you do not pass a callback function to toArray. The promise resolves to the array of pieces (use then).

That code will fetch only pieces this particular user is allowed to see, and also take into account your custom mongodb criteria object and a full-text search for the word blue, sorting the results by how well they match that search.

# Updating pieces

You can also update a piece fetched in this way, again taking permissions into account:

piece.age = 60;
// Only succeeds if this user has the right to update this piece
return self.update(req, piece, callback);

There is an insert method as well, taking the same arguments. The new piece should be created with newInstance:

var piece = self.newInstance();
piece.title = 'So great!';
return self.insert(req, piece, callback);

This will only succeed if the current user should be allowed to create pieces of this type, based on the permissions of the groups they are a member of.

You may also use promises. If you do not pass the callback argument, Apostrophe returns a promise (use then).

self.update and self.insert also call self.beforeUpdate and self.beforeInsert, as well as self.beforeSave (called by both), which provides an easy way to add extra behavior on every save operation.

# Properties with a leading _ are special

One important rule: any property with a leading _, except for _id, will not be saved to the database.

Apostrophe reserves properties starting with _ for dynamically loaded information, such a the results of a join, or the ._url property. These properties can change at any time and duplicate information stored elsewhere in the database. Storing them back to the database would just cause confusion and waste space.

# Working with projections

You can limit the amount of data returned just like you would with MongoDB: using the second argument.

return self.find(req, { age: { $gte: 50 }}, { title: 1, tags: 1 })
  .toArray(function(err, pieces) { ... })

This projection restricts the results to the title and tags properties only. Including tags is a good idea if you want the ._url property of each piece to automatically be populated, taking into account the best match with an existing pieces-page on the site (i.e. a blog that is configured to show documents with those tags).

Working with projections is great for performance. However, do not use projections if you plan to save the doc back to the database. You will lose information permanently. Use them for read-only operations only.

You can also set the projection with the chainable .projection({...}) filter method.

# Fetching pieces from a different module

If your code is in a different module, then self.find won't be the right method. Instead, ask the apostrophe-docs module to give you the right module to talk to:

return self.apos.docs.getManager('candy').find(...)

If there's a subclass of pieces with its name option set to candy, then this call will return it. (Note: this is not the same thing as the name of the module. It is the name, usually singular, of one individual piece in the database, as found in the type property of each piece.)

Working with the right manager in this way ensures you get the benefit of any extra behavior that may be built into cursors, insert and update for this particular type.

# More about cursors

As you may have noticed, Apostrophe's cursors — the objects returned by find() — are pretty great. Check out the reference documentation for more information about cursors. TODO: give examples of adding a refinment to an existing cursor moog type to add more filters correctly. See the apostrophe-pieces source

# Working with pages

apos.pages has a find method too. It's very similar, but the cursors that it returns support some very useful chainable filter methods specific to pages, like ancestors(true) and children(true), which attach an array of _ancestors and an array of _children to each page retrieved.

return self.apos.pages.find(req, { slug: '/about' })
  .toArray(function(err, pages) {
    // Each page in `pages` has `._ancestors` and `._children` properties

You can even pass an object to each of these filters, in which case it is used to invoke filters on the cursor used to get the ancestors or children.

apos.pages also provides an insert method, which takes additional arguments to add the page to the page tree:

var child = apos.pages.newChild(parentPage);
child.title = 'something nifty';
return apos.pages.insert(req, parentPage, newPage, function(err) { ... }

You should fetch the parent of the page first, to pass as parentPage.

# Fetching mixed doc types

Sometimes we're interested in many types of docs, both pieces and pages. We still want Apostrophe's permissions features, but we don't want to be locked down to one type of piece, or to just pages. The apostrophe-search module is a good example. It works like this:

return self.apos.docs.find(req).search('blue').toArray(function(err, docs) {

When you call docs.find in this way, pieces won't have a ._url property. For that you need to use the find method of the appropriate module.

The docs module also has insert and update methods. These work just like the insert and update methods of pieces, but they won't invoke the beforeInsert, etc. methods of pieces. So use the method of the appropriate module unless you're bypassing this intentionally.

# skip, limit, page and perPage: paginating results

If you just call toArray, you can get a lot of data. A lot. Especially if there are thousands of, for instance, blog posts. Fetching it all every time is not scalable.

You can use skip and limit like you would with raw MongoDB:

return self.find(req).skip(100).limit(10).toArray(function(err, pages) { ... })

And you can use toCount to figure out how many documents there are in total:

return self.find(req).toCount(function(err, count) { ... })

But, consider using the built-in perPage and page filters, which are simpler:

return self.find(req).perPage(10).page(11).toArray(function(err, pages) { ... })

Tip: the apostrophe-pieces-pages module uses all this stuff, and it's pretty easy to follow. Check out the source code. And consider whether you should just be using it to display your pieces. It's pretty flexible and extensible.

# Adding special behavior when a piece is saved

To change Apostrophe's behavior when a particular type of piece is saved, just override the beforeSave method in your code for that module:

var superBeforeSave = self.beforeSave;
self.beforeSave = function(req, piece, options, callback) {
  piece.title = piece.firstName + ' ' + piece.lastName;
  return superBeforeSave(req, piece, options, callback);

This works for all insert and update operations that go through the module's insert and update operations, which includes all of the normal UI for editing pieces.

Here I'm invoking the original, "super" or "parent" version of the method after my own, to make sure I don't lose any existing useful beforeSave behavior. If I'm subclassing pieces directly, I know it's empty, so I can skip that bit and just invoke the callback.

# Adding special behavior when anything is saved

You can also extend Apostrophe's behavior for saving all types of docs. This is really useful if you're implementing something like a custom search engine.

A custom search engine module might have an index.js file a little like this:

var coolSearch = require('made-up-search-engine');

module.exports = {
  construct: function(self, options) {
    self.docBeforeSave = function(req, doc, callback) {
      return coolSearch.addToIndex(doc.title, callback);

Here's the magic: Apostrophe will call docBeforeSave for every module that has one.

Note that the callback is optional. If your docBeforeSave handler doesn't need to do anything async, it can declare just req, doc as parameters.

Performance warning: docBeforeSave handlers should be as fast as possible. Always begin by asking, "is this doc any of my business?" Usually a peek at doc.type tells you. If the answer is no... just invoke the callback and return immediately!