title: Select relationship in SQLAlchemy query
body: <p>I have the following model:</p>
  
  <pre><code>class business_member(base_model):
      __tablename__ = "business_member"
  
      id = Column(Integer, primary_key=True)
      business_id = Column(Integer, ForeignKey("business.id"), nullable=False)
      member_id = Column(Integer, ForeignKey("user.id"), nullable=False)
      business_parts_id = Column(Integer, ForeignKey("business_parts.id"), nullable=False)
      is_admin = Column(Boolean, nullable=True, default=False)
      created_date = Column(DateTime, nullable=False)
      modified_date = Column(DateTime, nullable=True)
  
      __table_args__ = (UniqueConstraint("business_id", "member_id", name="business_member_uc"), )
      business = relationship("business", back_populates="members")
      member = relationship("user", back_populates="as_employee")
      part = relationship("business_parts", back_populates="employees")
  </code></pre>
  
  <p>And I converted the following raw SQL:</p>
  
  <pre><code>SELECT id, member_id, is_admin FROM business_member WHERE id IN (
  SELECT id FROM business WHERE business_id = (
  SELECT business_id FROM business_member WHERE member_id = 5 and business_id = 1))
  </code></pre>
  
  <p>to SQLAlchemy query:</p>
  
  <pre><code>session = database.get_session()
  
  user_validation_query = session.query(business_member.business_id).filter_by(
      member_id=kwargs["user_id"], 
      business_id=request.raw_args["business_id"]
  ).subquery()
  
  get_business_id_query = session.query(business_member).options(
      load_only(business_member.id)
  ).filter(business_member.business_id == user_validation_query).subquery()
  
  member_list = session.query(business_member).filter(
      business_member.id.in_(get_business_id_query)
  ).all()
  </code></pre>
  
  <p>Everything is ok, but I want to return just id, is_admin and member(relationship) columns in the query result, I tried different approaches like <code>load_only()</code> but none of the approaches worked properly.
  how I can do this!?</p>
  
  <p><strong>Update:</strong></p>
  
  <p>For example, I changed the query to:</p>
  
  <pre><code>member_list = session.query(business_member).options(
      load_only("id", "is_admin", "member")
  ).filter(
      business_member.id.in_(get_business_id_query)
  ).all()
  </code></pre>
  
  <p>and I got the following exception:
  <code>can't locate strategy for (('deferred', False), ('instrument', True))</code></p>