title: Using md-select, pass the entire object of an ng-repeat item, not just a string
body: <p>I have made a form with Angular Material that uses a dropdown list to choose from an array of <code>book</code> objects, with names and IDs, for the ultimate purpose of trading it for another book in my collection. Here is what my typical trade object looks like:</p>
  
  <pre><code>var tradeItem = {
    wanted: {
      user: 'barryjohnson',
      bookId: 'foo1bar2'
    },
    offered: {
      user: 'dangraham',
      bookId: 'bar7baz8'
    },
  };
  </code></pre>
  
  <p>I want to save the entire object to my <code>ng-model</code> in my select menu, so I want the ng-repeat to refer to the entire array object. This way I can make a simple subsequent request to my database. </p>
  
  <p>At the moment it is only saving the title, which is what have inside my <code>md-option</code> tag.</p>
  
  <p>Here is my view:</p>
  
  <pre><code>md-dialog(aria-label='Trade', ng-cloak='')
    form
      md-toolbar
        .md-toolbar-tools
          h2 Propose a Trade
          span(flex='')
      md-dialog-content(style='max-width:800px;max-height:810px; ')
        .md-dialog-content
          p Which of your books would you like to trade for '{{userClickedBook.title}}'?
          p {{selected}}
          md-input-container.md-block
            label(for="mySelect") Your Books
            md-select(name="mySelect", ng-model='selected')
              md-option(ng-repeat="book in usersBooks") {{book.title}}
          div(layout='row')
            md-button(ng-click='cancelDialog()', style='margin-right: 20px;') Cancel
            md-button.md-primary(ng-click='save()', ng-disabled='myForm.$invalid', layout='', layout-align='center end') Ok
  </code></pre>
  
  <p>So, how can I bind the entire object, so that <code>$scope.selected</code> is like the above object <code>tradeItem</code>, instead of just a single string (the content of <code>md-option</code>)?</p>