title: Api model /child properties not seen in swagger ui
body: <p>I have classes PET and CATEGORY. PET HAS A category object. However when i run the application, I do not see fields of Category class in Swagger UI documentation
  I am not using spring and don't want to use spring. I am not sure what is missing ?</p>
  
  <pre><code>@ApiModel(value = "A pet is a person's best friend" )
  public class Pet {
    private long id;
    private Category category;
  
  public long getId() {
      return id;
    }
  
  
    @ApiModelProperty(value = "ID", required=true)
    public void setId(long id) {
      this.id = id;
    }
  
  
    public Category getCategory() {
      return category;
    }
  
  
    @ApiModelProperty(value = "CAT", required=true)
    public void setCategory(Category category) {
      this.category = category;
    }
  </code></pre>
  
  <p>AND CLASS CATEGORY as</p>
  
  <pre><code>@ApiModel(value = "Category")
  public class Category {
    private long id;
    private String name;
  
  
  
  
    @ApiModelProperty(value = "ID", required=true)
    public long getId() {
      return id;
    }
  
  
    public void setId(long id) {
      this.id = id;
    }
  </code></pre>
  
  <p>OUTPUT SWAGGER UI documentation</p>
  
  <pre><code>{
    "id": 0,
    "category": {}
  }
  </code></pre>
  
  <p>MISSING</p>
  
  <p>id is missing</p>
  
  <pre><code>  "category": {
   "id": 0
  }
  </code></pre>