title: SQLAlchemy: Building query with custom field and value
body: <p>How to convert following SQL query to SQLAlchemy analog?</p>
  
  <pre><code>SELECT 'custom_value' AS `custom_field`, f.`bar` FROM `Foo` f
  </code></pre>
  
  <p>'Foo' model:</p>
  
  <pre><code>from sqlalchemy.ext.declarative import declarative_base
  from sqlalchemy import Column, Integer, String
  
  Base = declarative_base()
  
  class Foo(Base):
      __tablename__ = 'Foo'
  
      id = Column(Integer, primary_key=True)
      bar = Column(String)
  </code></pre>