title: SqlAlchemy Error on Creating multiple foreign key to one table
body: <p>I am new using sqlAlchemy and having problem creating new tables, specially when it comes around 2 foreign keys pointing to 1 table:</p>
  
  <pre><code>class Offers(db.Model):
      __tablename__ = 'offers'
      id = db.Column(db.Integer, primary_key=True, autoincrement=True)
      contact_ign = db.Column(db.String(100))
      conversion_rate = db.Column(db.Float)
      stock = db.Column(db.Integer)
      create_date = db.Column(db.DateTime(timezone=True), default=func.now())
      currency_pair = db.relationship('CurrencyPairs', backref='pair', lazy='dynamic')
  
  class CurrencyPairs(db.Model):
      __tablename__ = 'currency_pairs'
      id = db.Column(db.Integer, primary_key=True, autoincrement=True)
      league = db.Column(db.String(100))
      pair_id = db.Column(db.Integer, db.ForeignKey('offers.id'))
      want = db.relationship('Currency', backref='want', lazy='dynamic')
      have = db.relationship('Currency', backref='have', lazy='dynamic')
  
  class Currency(db.Model):
      __tablename__ = 'currency'
      id = db.Column(db.Integer, primary_key=True, autoincrement=True)
      name = db.Column(db.String(100), nullable=False)
      poe_trade = db.Column(db.Integer, nullable=False)
      poe_official = db.Column(db.String(10), nullable=False)
      tier = db.Column(db.Integer, nullable=False)
      want_id = db.Column(db.Integer, db.ForeignKey('currency_pairs.id'))
      have_id = db.Column(db.Integer, db.ForeignKey('currency_pairs.id'))
  </code></pre>
  
  <p>The error I am getting is:</p>
  
  <pre><code>sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Triggering mapper: 'Mapper|CurrencyPairs|currency_pairs'. Original exception was: Could not determine join condition b
  etween parent/child tables on relationship CurrencyPairs.want - there are multiple foreign key paths linking the tables.  Specify the 'foreign_keys' argument, providing a list of those columns which should be counted as containing a foreign key refe
  rence to the parent table
  </code></pre>
  
  <p>I have try different things but I get same result.
  What am I doing wrong? 
  Thanks In advance.</p>