FastAPI response model with computed fields and excludes

<p>I have a SQLAlchemy ORM model with sensitive fields (api_key_hash, internal_flags) that I don't want to expose in API responses. I also want to add computed fields to the response (e.g., is_verified combining multiple conditions). I'm using Pydantic v2.</p>

<p>Use Pydantic response models with <code>from_orm</code> and <code>@computed_field</code>:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">pydantic</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseModel</span><span class="p">,</span> <span class="n">ConfigDict</span><span class="p">,</span> <span class="n">computed_field</span> <span class="kn">from</span><span class="w"> </span><span class="nn">datetime</span><span class="w"> </span><span class="kn">import</span> <span class="n">datetime</span> <span class="kn">import</span><span class="w"> </span><span class="nn">uuid</span> <span class="k">class</span><span class="w"> </span><span class="nc">TraceResponse</span><span class="p">(</span><span class="n">BaseModel</span><span class="p">):</span> <span class="n">model_config</span> <span class="o">=</span> <span class="n">ConfigDict</span><span class="p">(</span><span class="n">from_attributes</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="c1"># Enable ORM mode</span> <span class="nb">id</span><span class="p">:</span> <span class="n">uuid</span><span class="o">.</span><span class="n">UUID</span> <span class="n">title</span><span class="p">:</span> <span class="nb">str</span> <span class="n">context_text</span><span class="p">:</span> <span class="nb">str</span> <span class="n">solution_text</span><span class="p">:</span> <span class="nb">str</span> <span class="n">status</span><span class="p">:</span> <span class="nb">str</span> <span class="n">trust_score</span><span class="p">:</span> <span class="nb">float</span> <span class="n">created_at</span><span class="p">:</span> <span class="n">datetime</span> <span class="c1"># NOTE: api_key_hash, is_flagged, internal_fields NOT included</span> <span class="nd">@computed_field</span> <span class="nd">@property</span> <span class="k">def</span><span class="w"> </span><span class="nf">is_validated</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">bool</span><span class="p">:</span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="s1">'validated'</span> <span class="c1"># For nested models:</span> <span class="k">class</span><span class="w"> </span><span class="nc">TagResponse</span><span class="p">(</span><span class="n">BaseModel</span><span class="p">):</span> <span class="n">model_config</span> <span class="o">=</span> <span class="n">ConfigDict</span><span class="p">(</span><span class="n">from_attributes</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="nb">id</span><span class="p">:</span> <span class="n">uuid</span><span class="o">.</span><span class="n">UUID</span> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span> <span class="k">class</span><span class="w"> </span><span class="nc">TraceWithTagsResponse</span><span class="p">(</span><span class="n">TraceResponse</span><span class="p">):</span> <span class="n">tags</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="n">TagResponse</span><span class="p">]</span> <span class="o">=</span> <span class="p">[]</span> <span class="c1"># In route handler:</span> <span class="nd">@router</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'/traces/</span><span class="si">{trace_id}</span><span class="s1">'</span><span class="p">,</span> <span class="n">response_model</span><span class="o">=</span><span class="n">TraceWithTagsResponse</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_trace</span><span class="p">(</span><span class="n">trace_id</span><span class="p">:</span> <span class="n">uuid</span><span class="o">.</span><span class="n">UUID</span><span class="p">,</span> <span class="n">db</span><span class="p">:</span> <span class="n">DbSession</span><span class="p">):</span> <span class="n">trace</span> <span class="o">=</span> <span class="k">await</span> <span class="n">get_trace_with_tags</span><span class="p">(</span><span class="n">db</span><span class="p">,</span> <span class="n">trace_id</span><span class="p">)</span> <span class="k">return</span> <span class="n">trace</span> <span class="c1"># FastAPI serializes via response_model automatically</span> </code></pre></div> <p>Key points: - <code>from_attributes=True</code> (Pydantic v2) replaces <code>orm_mode=True</code> (Pydantic v1) - Only fields declared in the response model are included — sensitive fields are excluded by default - <code>@computed_field</code> computes dynamic fields at serialization time - Use <code>response_model_exclude_unset=True</code> in the route to exclude None fields</p>