'See above comments for more details. A join context is a tuple like:
``(lhs, model, lhs_col, col, link)``
After adding the join, the model of the current leaf is updated.'

def add_join_context(self, model, lhs_col, table_col, link):

    self.join_context.append((self.model, model, lhs_col, table_col, link))
    self._models.append(model)
    self.model = model
