SQLAlchemy ORM check if column is a foreign_key

Question

Hi I was wondering if anyone knows of the most efficient way to find out if a column has a foreignKey relationship or not.

Answer 1

I had a similar problem. Actually I didn't have the table object, but the table name, and similarly the column name. Here is my solution:

from sqlalchemy.schema import MetaData
from sqlalchemy import create_engine

engine = create_engine( URL )
meta = MetaData()
meta.reflect(bind=engine)

def is_foreign_key(table_name, col_name):
    return col_name in [e.target_fullname for e in meta.tables[table_name].foreign_keys]

NOTE:

meta.tables[table_name].foreign_keys == Table.__table__.foreign_keys

Answer 2

The ForeignKey objects associated with an individual Column object are available in the foreign_keys collection of that column.

foreign_keys_set = StudentIntendCourse.__table__.c.studentID.foreign_keys

You can check if this set is non empty

Answer 3

All columns have the attribute 'foreign_keys', that is a set, but it is empty for all column that are not ForeignKey, so a simple test can give you the correct answer. In your example:

def is_foreign_key(column):
    return True if column.foreign_keys else False

print(is_foreign_key(StudentIntendCourse.studentID))

If you want to have list of all ForeignKey:

from sqlalchemy.inspection import inspect

foreign_keys_list = [c.key for c in inspect(StudentIntendCourse) if getattr(StudentIntendCourse, c.key).foreign_keys]

I strongly suggest to use inspection, instead, directly access the nested structures of SQLAlchemy.