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                    Migration
            
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                    find()
            
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                    findByIds$()
            
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                    importJSON()
            
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                    syncCouchDB()
            
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                    syncGraphQL()
            
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                    remove()
            
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                    destroy()
            
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                    isRxCollection()
            
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                    RxDocument
            
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                    Insert
            
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                    Find
            
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                    Functions
            
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                    get()
            
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                    get$()
            
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                    proxy-get
            
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                    update()
            
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                    atomicUpdate()
            
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                    atomicPatch()
            
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                    $
            
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                    remove()
            
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                    deleted$
            
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                    toJSON()
            
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                    set()
            
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                    save()
            
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                    isRxDocument()
            
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                    RxQuery
            
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                    find()
            
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                    findOne()
            
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                    exec()
            
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                    $
            
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                    update()
            
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                    remove()
            
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                    doesDocumentDataMatch()
            
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                    Specific Index
            
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                    Examples
            
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                    isRxQuery()
            
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                    RxAttachment
            
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                    Middleware-hooks
            
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                    ORM/DRM
            
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                    Population
            
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                    Encryption
            
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                                <h1 id="datamigration">DataMigration</h1>
<p>Imagine you have your awesome messenger-app distributed to many users. After a while, you decide that in your new version, you want to change the schema of the messages-collection. Instead of saving the message-date like <code>2017-02-12T23:03:05+00:00</code> you want to have the unix-timestamp like <code>1486940585</code> to make it easier to compare dates. To accomplish this, you change the schema and increase the version-number and you also change your code where you save the incoming messages. But one problem remains: what happens with the messages which are already saved on the user&apos;s device in the old schema?</p>
<p>With RxDB you can provide migrationStrategies for your collections that automatically (or on call) transform your existing data from older to newer schemas. This assures that the client&apos;s data always matches your newest code-version.</p>
<h1 id="add-the-migration-plugin">Add the migration plugin</h1>
<p>To enable the data migration, you have to add the <code>migration</code> plugin.</p>
<pre><code class="lang-ts"><span class="hljs-keyword">import</span> { addRxPlugin } from <span class="hljs-string">&apos;rxdb&apos;</span>;
<span class="hljs-keyword">import</span> { RxDBMigrationPlugin } from <span class="hljs-string">&apos;rxdb/plugins/migration&apos;</span>;
addRxPlugin(RxDBMigrationPlugin);
</code></pre>
<h2 id="providing-strategies">Providing strategies</h2>
<p>Upon creation of a collection, you have to provide migrationStrategies when your schema&apos;s version-number is greater than <code>0</code>. To do this, you have to add an object to the <code>migrationStrategies</code> property where a function for every schema-version is assigned. A migrationStrategy is a function which gets the old document-data as a parameter and returns the new, transformed document-data. If the strategy returns <code>null</code>, the document will be removed instead of migrated.</p>
<pre><code class="lang-javascript">myDatabase.addCollections({
  messages: {
    schema: messageSchemaV1,
    migrationStrategies: {
      <span class="hljs-comment">// 1 means, this transforms data from version 0 to version 1</span>
      <span class="hljs-number">1</span>: <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        oldDoc.time = <span class="hljs-keyword">new</span> <span class="hljs-built_in">Date</span>(oldDoc.time).getTime(); <span class="hljs-comment">// string to unix</span>
        <span class="hljs-keyword">return</span> oldDoc;
      }
    }
  }
});
</code></pre>
<p>Asynchronous strategies can also be used:</p>
<pre><code class="lang-javascript">myDatabase.addCollections({
  messages: {
    schema: messageSchemaV1,
    migrationStrategies: {
      <span class="hljs-number">1</span>: <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        oldDoc.time = <span class="hljs-keyword">new</span> <span class="hljs-built_in">Date</span>(oldDoc.time).getTime(); <span class="hljs-comment">// string to unix</span>
        <span class="hljs-keyword">return</span> oldDoc;
      },
      <span class="hljs-comment">/**
       * 2 means, this transforms data from version 1 to version 2
       * this returns a promise which resolves with the new document-data
       */</span>
      <span class="hljs-number">2</span>: <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        <span class="hljs-comment">// in the new schema (version: 2) we defined &apos;senderCountry&apos; as required field (string)</span>
        <span class="hljs-comment">// so we must get the country of the message-sender from the server</span>
        <span class="hljs-keyword">const</span> coordinates = oldDoc.coordinates;
        <span class="hljs-keyword">return</span> fetch(<span class="hljs-string">&apos;http://myserver.com/api/countryByCoordinates/&apos;</span>+coordinates+<span class="hljs-string">&apos;/&apos;</span>)
          .then(response =&gt; {
            <span class="hljs-keyword">const</span> response = response.json();
            oldDoc.senderCountry=response;
            <span class="hljs-keyword">return</span> oldDoc;
          });
      }
    }
  }
});
</code></pre>
<p>you can also filter which documents should be migrated:</p>
<pre><code class="lang-js">myDatabase.addCollections({
  messages: {
    schema: messageSchemaV1,
    migrationStrategies: {
      <span class="hljs-comment">// 1 means, this transforms data from version 0 to version 1</span>
      <span class="hljs-number">1</span>: <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        oldDoc.time = <span class="hljs-keyword">new</span> <span class="hljs-built_in">Date</span>(oldDoc.time).getTime(); <span class="hljs-comment">// string to unix</span>
        <span class="hljs-keyword">return</span> oldDoc;
      },
      <span class="hljs-comment">/**
       * this removes all documents older then 2017-02-12
       * they will not appear in the new collection
       */</span>
      <span class="hljs-number">2</span>: <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        <span class="hljs-keyword">if</span>(oldDoc.time &lt; <span class="hljs-number">1486940585</span>) <span class="hljs-keyword">return</span> <span class="hljs-literal">null</span>;
        <span class="hljs-keyword">else</span> <span class="hljs-keyword">return</span> oldDoc;
      }
    }
  }
});
</code></pre>
<h2 id="automigrate">autoMigrate</h2>
<p>By default, the migration automatically happens when the collection is created. If you have lots of data or the migrationStrategies take a long time, it might be better to start the migration &apos;by hand&apos; and show the migration-state to the user as a loading-bar.</p>
<pre><code class="lang-javascript"><span class="hljs-keyword">const</span> messageCol = <span class="hljs-keyword">await</span> myDatabase.addCollections({
  messages: {
    schema: messageSchemaV1,
    autoMigrate: <span class="hljs-literal">false</span>, <span class="hljs-comment">// &lt;- migration will not run at creation</span>
    migrationStrategies: {
      <span class="hljs-number">1</span>: <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        ...
        anything that takes very long
        ...
        return oldDoc;
      }
    }
  }
});

<span class="hljs-comment">// check if migration is needed</span>
<span class="hljs-keyword">const</span> needed = <span class="hljs-keyword">await</span> messageCol.migrationNeeded();
<span class="hljs-keyword">if</span>(needed == <span class="hljs-literal">false</span>) <span class="hljs-keyword">return</span>;

<span class="hljs-comment">// starting the migration</span>

<span class="hljs-keyword">const</span> migrationState$ = messageCol.migrate(<span class="hljs-number">10</span>); <span class="hljs-comment">// 10 is the batch-size, how many docs will run at parallel</span>

<span class="hljs-comment">// &apos;start&apos; the observable</span>
migrationState$.subscribe(
  state =&gt; <span class="hljs-built_in">console</span>.dir(state),
  error =&gt; <span class="hljs-built_in">console</span>.error(error),
  done =&gt; <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&apos;done&apos;</span>)
);

<span class="hljs-comment">// the emitted states look like this:</span>
{
    done: <span class="hljs-literal">false</span>, <span class="hljs-comment">// true if finished</span>
    total: <span class="hljs-number">50</span>,   <span class="hljs-comment">// amount of documents which must be migrated</span>
    handled: <span class="hljs-number">0</span>,  <span class="hljs-comment">// amount of handled docs</span>
    success: <span class="hljs-number">0</span>,  <span class="hljs-comment">// handled docs which succeeded</span>
    deleted: <span class="hljs-number">0</span>,  <span class="hljs-comment">// handled docs which got deleted</span>
    percent: <span class="hljs-number">0</span>   <span class="hljs-comment">// percentage</span>
}
</code></pre>
<p>If you don&apos;t want to show the state to the user, you can also use <code>.migratePromise()</code>:</p>
<pre><code class="lang-js">  <span class="hljs-keyword">const</span> migrationPromise = messageCol.migratePromise(<span class="hljs-number">10</span>);
  <span class="hljs-keyword">await</span> migratePromise;
</code></pre>
<h2 id="migrationstates">migrationStates()</h2>
<p><code>RxDatabase.migrationStates()</code> returns an <code>Observable</code> that emits all events to the ongoing migrations of all <code>RxCollection</code>s. Use it to log the current state of the overall migration process or to show a loading state to your user.</p>
<pre><code class="lang-js"><span class="hljs-keyword">const</span> allStatesObservable = myDatabase.migrationStates();
allStatesObservable.subscribe(allStates =&gt; {
  allStates.forEach(collectionState =&gt; {
    <span class="hljs-built_in">console</span>.log(
      <span class="hljs-string">&apos;migration state of &apos;</span> +
      collectionState.collection.name + <span class="hljs-string">&apos;: &apos;</span> +
      collectionState.state.done
    );
  });
});
</code></pre>
<h2 id="migrating-attachments">Migrating attachments</h2>
<p>When you store <code>RxAttachment</code>s together with your document, they can also be changed, added or removed while running the migration.
You can do this by mutating the <code>oldDoc._attachments</code> property.</p>
<pre><code class="lang-js"><span class="hljs-keyword">import</span> { blobBufferUtil } <span class="hljs-keyword">from</span> <span class="hljs-string">&apos;rxdb&apos;</span>;
<span class="hljs-keyword">const</span> migrationStrategies = {
      <span class="hljs-number">1</span>: <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        <span class="hljs-comment">// do nothing with _attachments to keep all attachments and have them in the new collection version.</span>
        <span class="hljs-keyword">return</span> oldDoc;
      },
      <span class="hljs-number">2</span>: <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        <span class="hljs-comment">// set _attachments to an empty object to delete all existing ones during the migration.</span>
        oldDoc._attachments = {};
        <span class="hljs-keyword">return</span> oldDoc;
      }
      <span class="hljs-number">3</span>: <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">function</span>(<span class="hljs-params">oldDoc</span>)</span>{
        <span class="hljs-comment">// update the data field of a single attachment to change its data. </span>
        oldDoc._attachments.myFile.data = <span class="hljs-keyword">await</span> blobBufferUtil.createBlobBuffer(
          <span class="hljs-string">&apos;my new text&apos;</span>,
          oldDoc._attachments.myFile.content_type
        );
        <span class="hljs-keyword">return</span> oldDoc;
      }
}
</code></pre>
<h2 id="hint">Hint</h2>
<p>If your migration takes a long time, combine it with the leaderElection to make sure you don&apos;t waste your users&apos; resources by running it in 2 open tabs.</p>

                                
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