controlador.php

   public function all()
    {
      $c = new compra();
      $compras = $c->get_compras() ;
      $data = array();
      foreach ($compras as $com) {
        $fecha = date_create($com['fecha']);//funcion para voltear la fecha
        $row = array($com['codigo'],
          $com['proveedor'],
        date_format($fecha, 'd-m-Y'),//se voltea la fecha
        $com['monto'],
        '<a  data-toggle="tooltip" title="Ver" href="javascript:void(0)" onclick=""><i class=" btn btn-xs btn-ver
        glyphicon glyphicon-eye-open"></i></a>
        <a  data-toggle="tooltip" title="Eliminar" href="javascript:void(0)" onclick="" ><i class=" btn btn-xs btn-delete
          glyphicon glyphicon-trash"></i></a>');
        $data[] = $row;
      }
      $output = array("data" => $data);
      echo json_encode($output);
    }

modelo.php

 public function get_compras()
  {
    try
    {
      $query = $this->dbh->prepare('
        SELECT
        c.cod_compra AS codigo,
        p.razon_social AS proveedor,
        c.fecha_actual AS fecha,
        c.tot AS monto
        FROM
        compras AS c
        INNER JOIN
        proveedores AS p
        ON c.id_prov = p.id_prov
        ORDER BY
        c.cod_compra
        DESC
        ');
      $query->execute();
      return $query->fetchAll();
      $this->dbh = null;
    }catch (PDOException $e)
    {
      $e->getMessage();
    }
  }

vista.php

<section class="content-header">
    <h1>
        Listado de compras
        <small></small>
    </h1>
    <ol class="breadcrumb">
        <a  href="views/reportes/todas_las_compras.php" role="button" class="btn btn-delete"><span class="fa fa-file-pdf-o"></span> Imprimir</a>
    </section>
    <hr>
    <section class="content">
          <div class="row">
            <div class="col-xs-12">
              <div class="box box-primary">
                <div class="box-header">
                  <h3 class="box-title"><strong>Listado de compras</strong></h3>
                </div><!-- /.box-header -->
                <div class="box-body">
                  <table id="table" class="table table-bordered table-striped">
                    <thead>
                      <tr>
                        <th>Codigo</th>
                        <th>Proveedor</th>
                        <th>Fecha</th>
                        <th>Monto</th>
                        <th style="width: 80px;" >Acci&oacuten</th>
                      </tr>
                    </thead>
                  </table>
                </div><!-- /.box-body -->
              </div><!-- /.box -->
            </div><!-- /.col -->
          </div><!-- /.row -->
        </section><!-- /.content -->
      </div>
<script type="text/javascript">
    var table;
    $(document).ready(function() {
//datatables
table = $('#table').DataTable({
    "scrollX": true,
    "processing": true, //Feature control the processing indicator.
    "ajax": {
       "url": "?controller=compras&action=all",
       "type": "POST"
   },
});
});
function recargar(){
    table.ajax.reload(null,false); //reload datatable ajax
}
</script>