AttributeError: 'SQLAlchemy' object has no attribute 'FieldType'

Python, flask and sqlalchemy newbies here. I am writing a small application to collect data (from an excel file) from different groups (departments). Each department will have associated tables and associated fields. Fields will have predefined types (number, regular text, date). My models.py is:

from app import db
from sqlalchemy.schema import Sequence, CreateSequence

class FieldType(db.Model):

    __tablename__ = 'fieldtype'

    id = db.Column(db.Integer, db.Sequence('ftypeid'), primary_key = True)
    name = db.Column(db.String(120))
    python_type = db.Column(db.String(120))
    python_import = db.Column(db.String(120))

class Field(db.Model):

    __tablename__ = 'field'

    id = db.Column(db.Integer, db.Sequence('fieldid'), primary_key = True)
    name = db.Column(db.String(120), index = True, unique = True)
    position = db.Column(db.Integer)
    ftype = db.Column(db.FieldType) #The error is happening here.
    tableinfoid = db.Column(db.Integer, db.ForeignKey('tableinfo.id'))

class TableInfo(db.Model):
    __tablename__ = 'tableinfo'

    id = db.Column(db.Integer, db.Sequence('tinfoid'), primary_key = True)
    name = db.Column(db.String(30), index = True, unique = True)
    fields = db.relationship('Field', backref='parent_table', lazy='dynamic')

class Department(db.Model):
    __tablename__ = 'department'

    id = db.Column(db.Integer, db.Sequence('dpartmntid'), primary_key = True)
    name = db.Column(db.String(120), index = True, unique = True)

When I try to run the app I get the error:

(env)[vagrant@localhost dataupload]$ ./run.py
Traceback (most recent call last):
File "./run.py", line 2, in <module>
from app import theapp
File "/home/vagrant/Development/dataupload/app/__init__.py", line 13, in <module>
from app import views, models
File "/home/vagrant/Development/dataupload/app/models.py", line 13, in <module>
class Field(db.Model):
File "/home/vagrant/Development/dataupload/app/models.py", line 20, in Field
ftype = db.Column(db.FieldType)     #The error is happening here.
AttributeError: 'SQLAlchemy' object has no attribute 'FieldType'

I am using my understanding of object oriented design. But I may not fully understand how sqlalchemy does the mapping - I followed Miguel's Flask Mega-Tutorial closely. But I think I'll start defining the field types and go from there. If there is a gap in my understanding, please help me fill it so I can complete this task. Thanks in advance.