title: How to define a table without primary key with SQLAlchemy?
body: <p>I have a table that does not have a <code>primary key</code>. And I really do not want to apply this constraint to this table.</p>
  
  <p>In <em>SQLAlchemy</em>, I defined the table class by:</p>
  
  <pre><code>class SomeTable(Base):
    __table__ = Table('SomeTable', meta, autoload=True, autoload_with=engine)
  </code></pre>
  
  <p>When I try to query this table, I got:</p>
  
  <pre><code>ArgumentError: Mapper Mapper|SomeTable|SomeTable could not assemble any primary key columns for mapped table 'SomeTable'.
  </code></pre>
  
  <p>How to loss the constraint that every table must have a primary key?</p>