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###Database
Nylas Mail is built on top of a custom database layer modeled after ActiveRecord. For many parts of the application, the database is the source of truth. Data is retrieved from the API, written to the database, and changes to the database trigger Stores and components to refresh their contents. The illustration below shows this flow of data:
The Database connection is managed by the DatabaseStore, a singleton object that exists in every window. All Database requests are asynchronous. Queries are forwarded to the application's Browser process via IPC and run in SQLite.
#####Declaring Models
In Nylas Mail, Models are thin wrappers around data with a particular schema. Each Model class declares a set of attributes that define the object's data. For example:
class Example extends Model
@attributes:
'id': Attributes.String
queryable: true
modelKey: 'id'
'object': Attributes.String
modelKey: 'object'
'namespaceId': Attributes.String
queryable: true
modelKey: 'namespaceId'
jsonKey: 'namespace_id'
'body': Attributes.JoinedData
modelTable: 'MessageBody'
modelKey: 'body'
'files': Attributes.Collection
modelKey: 'files'
itemClass: File
'unread': Attributes.Boolean
queryable: true
modelKey: 'unread'
When models are inflated from JSON using fromJSON or converted to JSON using toJSON, only the attributes declared on the model are copied. The modelKey and jsonKey options allow you to specify where a particular key should be found. Attributes are also coerced to the proper types: String attributes will always be strings, Boolean attributes will always be true or false, etc. null is a valid value for all types.
The DatabaseStore automatically maintains cache tables for storing Model objects. By default, models are stored in the cache as JSON blobs and basic attributes are not queryable. When the queryable option is specified on an attribute, it is given a separate column and index in the SQLite table for the model, and you can construct queries using the attribute:
Thread.attributes.namespaceId.equals("123")
// where namespace_id = '123'
Thread.attributes.lastMessageTimestamp.greaterThan(123)
// where last_message_timestamp > 123
Thread.attributes.lastMessageTimestamp.descending()
// order by last_message_timestamp DESC
#####Retrieving Models
You can make queries for models stored in SQLite using a Promise-based ActiveRecord-style syntax. There is no way to make raw SQL queries against the local data store.
DatabaseStore.find(Thread, '123').then (thread) ->
# thread is a thread object
DatabaseStore.findBy(Thread, {subject: 'Hello World'}).then (thread) ->
# find a single thread by subject
DatabaseStore.findAll(Thread).where([Thread.attributes.tags.contains('inbox')]).then (threads) ->
# find threads with the inbox tag
DatabaseStore.count(Thread).where([Thread.attributes.lastMessageTimestamp.greaterThan(120315123)]).then (results) ->
# count threads where last message received since 120315123.
#####Retrieving Pages of Models
If you need to paginate through a view of data, you should use a DatabaseView. Database views can be configured with a sort order and a set of where clauses. After the view is configured, it maintains a cache of models in memory in a highly efficient manner and makes it easy to implement pagination. DatabaseView also performs deep inspection of it's cache when models are changed and can avoid costly SQL queries.
#####Saving and Updating Models
The DatabaseStore exposes two methods for creating and updating models: persistModel and persistModels. When you call persistModel, queries are automatically executed to update the object in the cache and the DatabaseStore triggers, broadcasting an update to the rest of the application so that views dependent on these kind of models can refresh.
When possible, you should accumulate the objects you want to save and call persistModels. The DatabaseStore will generate batch insert statements, and a single notification will be broadcast throughout the application. Since saving objects can result in objects being re-fetched by many stores and components, you should be mindful of database insertions.
#####Saving Drafts
Drafts in Nylas Mail presented us with a unique challenge. The same draft may be edited rapidly by unrelated parts of the application, causing race scenarios. (For example, when the user is typing and attachments finish uploading at the same time.) This problem could be solved by object locking, but we chose to marshall draft changes through a central DraftStore that debounces database queries and adds other helpful features. See the DraftStore documentation for more information.
#####Removing Models
The DatabaseStore exposes a single method, unpersistModel, that allows you to purge an object from the cache. You cannot remove a model by ID alone - you must load it first.
####Advanced Model Attributes
#####Attribute.JoinedData
Joined Data attributes allow you to store certain attributes of an object in a separate table in the database. We use this attribute type for Message bodies. Storing message bodies, which can be very large, in a separate table allows us to make queries on message metadata extremely fast, and inflate Message objects without their bodies to build the thread list.
When building a query on a model with a JoinedData attribute, you need to call include to explicitly load the joined data attribute. The query builder will automatically perform a LEFT OUTER JOIN with the secondary table to retrieve the attribute:
DatabaseStore.find(Message, '123').then (message) ->
// message.body is undefined
DatabaseStore.find(Message, '123').include(Message.attributes.body).then (message) ->
// message.body is defined
When you call persistModel, JoinedData attributes are automatically written to the secondary table.
JoinedData attributes cannot be queryable.
#####Attribute.Collection
Collection attributes provide basic support for one-to-many relationships. For example, Threads in Nylas Mail have a collection of Tags.
When Collection attributes are marked as queryable, the DatabaseStore automatically creates a join table and maintains it as you create, save, and delete models. When you call persistModel, entries are added to the join table associating the ID of the model with the IDs of models in the collection.
Collection attributes have an additional clause builder, contains:
DatabaseStore.findAll(Thread).where([Thread.attributes.tags.contains('inbox')])
This is equivalent to writing the following SQL:
SELECT `Thread`.`data` FROM `Thread` INNER JOIN `Thread-Tag` AS `M1` ON `M1`.`id` = `Thread`.`id` WHERE `M1`.`value` = 'inbox' ORDER BY `Thread`.`last_message_timestamp` DESC
Listening for Changes
For many parts of the application, the Database is the source of truth. Funneling changes through the database ensures that they are available to the entire application. Basing your packages on the Database, and listening to it for changes, ensures that your views never fall out of sync.
Within Reflux Stores, you can listen to the DatabaseStore using the listenTo helper method:
@listenTo(DatabaseStore, @_onDataChanged)
Within generic code, you can listen to the DatabaseStore using this syntax:
@unlisten = DatabaseStore.listen(@_onDataChanged, @)
When a model is persisted or unpersisted from the database, your listener method will fire. It's very important to inspect the change payload before making queries to refresh your data. The change payload is a simple object with the following keys:
{
"objectClass": // string: the name of the class that was changed. ie: "Thread"
"objects": // array: the objects that were persisted or removed
}
But why can't I...?
Nylas Mail exposes a minimal Database API that exposes high-level methods for saving and retrieving objects. The API was designed with several goals in mind, which will help us create a healthy ecosystem of third-party packages:
- Package code should not be tightly coupled to SQLite
- Queries should be composed in a way that makes invalid queries impossible
- All changes to the local database must be observable