title: Filter page datasource based on record selection
body: <p>I seem to be struggling with this. I am working on a project management app in Google App Maker. I am using SQL data models. The portion of the app I am having trouble with is several project detail pages linked with a project list page. I have no trouble going from the project list page to the first project detail page.</p>
  
  <p>I have a table widget in the project list page, and the onClick code for a table row is:</p>
  
  <pre><code>    app.datasources.Projects.selectKey(widget.datasource.item._key);
      app.showPage(app.pages.ProjectPage);
  </code></pre>
  
  <p>This filters the first detail page with the primary key of the record in focus. No problem here. When I want to navigate the next detail page that has a different datasource (but I still need to filter using the same primary key) I am struggling.</p>
  
  <p>If someone can guide me in making a button in the first detail page that when clicked gets the _key of the project loaded in that current page and filters another detail page for the same project in focus, (using another data model, let's call it ProjectBudgetPage for example) I would really appreciate the help.</p>
  
  <p>This was one my latest attempts at navigation to other detail pages within a project, this was for an onClick event:</p>
  
  <pre><code>     var widgets = widget.parent.descendants;
       var projectID = widgets.HiddenProjectID.text;
       var projectDataSource = app.datasources.ProjectBudgetView.item;
       projectDataSource = projectID;
       app.showPage(app.pages.ProjectInfoPage);
  </code></pre>
  
  <p>which results in: <code>(TypeError) : Cannot read property '__gwt_instance' of undefined
  at HeaderProjectTabs.HeaderContainer.InfoTabButton.onClick:5:5</code></p>