Links and joints don't appear in generated SDF

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I'm using ROS and Gazebo and I'm a bit mystified by the URDF to SDF conversion process. It appears that all links except the base_link, and all joints, are not added to the generated SDF file. What I get is a single link with 1 combined inertial section and multiple collision and visual sections. Most of the names are mangled.

I'm writing a plugin that applies force to some joints, and I need to get the joint poses. I see plugin code that enumerates links and joints looking at name matches, but I suppose that this is only possible if I hand-build an SDF file? Is there a preferred way to manage this?

For reference, here's a snippet of my roslaunch file showing how I'm injecting URDF files into Gazebo:

<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-param robot_description -urdf -z 1 -model test"/>

Updated: here's a stripped down URDF file that I used for testing:

<?xml version="1.0"?>
<robot name="test">
  <link name="base_link">
    <inertial>
      <origin xyz="0 0 0"/>
      <mass value="1"/>
      <inertia
        ixx="1.0" ixy="0.0" ixz="0.0"
        iyy="1.0" iyz="0.0"
        izz="1.0"/>
    </inertial>
    <visual>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </collision>
  </link>
  <link name="two">
    <inertial>
      <origin xyz="0 0 0"/>
      <mass value="2"/>
      <inertia
        ixx="1.0" ixy="0.0" ixz="0.0"
        iyy="1.0" iyz="0.0"
        izz="1.0"/>
    </inertial>
    <visual>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </collision>
  </link>
  <joint name="base_link_to_two" type="fixed">
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <parent link="base_link"/>
    <child link="two"/>
  </joint>
</robot>

Here's the resulting SDF:

<sdf version='1.6'>
  <model name='test'>
    <link name='base_link'>
      <pose frame=''>0 0 0 0 -0 0</pose>
      <inertial>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <mass>3</mass>
        <inertia>
          <ixx>2</ixx>
          <ixy>0</ixy>
          <ixz>0</ixz>
          <iyy>2</iyy>
          <iyz>0</iyz>
          <izz>2</izz>
        </inertia>
      </inertial>
      <collision name='base_link_collision'>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
      </collision>
      <collision name='base_link_fixed_joint_lump__two_collision_1'>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
      </collision>
      <visual name='base_link_visual'>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
      </visual>
      <visual name='base_link_fixed_joint_lump__two_visual_1'>
        <pose frame=''>0 0 0 0 -0 0</pose>
        <geometry>
          <box>
            <size>1 1 1</size>
          </box>
        </geometry>
      </visual>
    </link>
  </model>
</sdf>