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| 20 | <h1 class="page-title">Tutorial: Changes API</h1>
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| 22 | <section>
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| 24 | <header>
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| 26 |
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| 27 | <h2>Changes API</h2>
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| 30 | <article>
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| 31 | <h1>Overview</h1><p>LokiJS 1.1 introduces a "Changes API" that enables the user to keep track of the changes happened to each collection since a particular point in time, which is usually the start of a work session but it could be a user defined one.
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| 32 | This is particularly useful for remote synchronization.</p>
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| 33 | <h2>Description of the Changes API</h2><p>The Changes API is a collection-level feature, hence you can establish which collections may simply contain volatile data and which ones need to keep a record of what has changed.</p>
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| 34 | <p>The Changes API is an optional feature and can be activated/deactivated by either passing the option <code>{ disableChangesApi: isDisabled }</code> in the config parameter of a collection constructor, or by calling <code>collection.setChangesApi(isEnabled)</code>.
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| 35 | Note that LokiJS will always set the fastest performing setting as default on a collection or database, hence the Changes API is <strong>disabled</strong> by default.</p>
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| 36 | <p>There are three events which will trigger a Changes API operation: inserts, updates and deletes.
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| 37 | When either of these events occur, on a collection with Changes API activated, the collection will store a snapshot of the relevant object, associated with the operation and the name of the collection.</p>
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| 38 | <p>From the database object it is then possible to invoke the <code>serializeChanges</code> method which will generate a string representation of the changes occurred to be used for synchronization purposes.</p>
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| 39 | <h2>Usage</h2><p>To enable the Changes API make sure to either instantiate a collection using <code>db.addCollection('users', { disableChangesApi: false })</code>, or call <code>users.setChangesApi(true)</code> (given an example <code>users</code> collection).</p>
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| 40 | <p>To generate a string representation of the changes, call <code>db.serializeChanges()</code>. This will generate a representation of all the changes for those collections that have the Changes API enabled. If you are only interested in generating changes for a subset of collections, you can pass an array of names of the collections, i.e. <code>db.serializeChanges(['users']);</code>.</p>
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| 41 | <p>To clear all the changes, call <code>db.clearChanges()</code>. Alternatively you can call <code>flushChanges()</code> on the single collection, normally you would call <code>db.clearChanges()</code> on a callback from a successful synchronization operation.</p>
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| 42 | <p>Each change is an object with three properties: <code>name</code> is the collection name, <code>obj</code> is the string representation of the object and <code>operation</code> is a character representing the operation ("I" for insert, "U" for update, "R" for remove). So for example, inserting user <code>{ name: 'joe' }</code> in the users collection would generate a change <code>{ name: 'users', obj: { name: 'joe' }, operation: 'I' }</code>. Changes are kept in order of how the happened so a 3rd party application will be able to operate insert updates and deletes in the correct order.</p>
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