SQLModel Session exercise¶

Adapted from Drew's slack example

In [1]:

from chowda.db import init_db
from tests.factories import *
init_db()

from chowda.db import init_db from tests.factories import * init_db()

Create a new Batch from factory¶

In [2]:

batch = BatchFactory.create()

batch = BatchFactory.create()

Grab the session the Batch was created with for inspection.¶

In [3]:

from sqlmodel import Session

session = Session.object_session(batch)

from sqlmodel import Session session = Session.object_session(batch)

Change something on the batch¶

In [4]:

batch.description = "exceedingly jolly"

batch.description = "exceedingly jolly"

View the changes that have yet to be committed to the db¶

In [5]:

print('session.dirty = ', session.dirty)
# => session.dirty = IdentitySet([Batch(description='foo')])

print('session.dirty = ', session.dirty) # => session.dirty = IdentitySet([Batch(description='foo')])

session.dirty =  IdentitySet([Batch(description='exceedingly jolly')])

Commit the changes to the db.¶

Note that we do not have to call session.add(batch) since the Batch instance is already attached to the session.

In [6]:

session.commit()

session.commit()

Verify there are no other pending changes.¶

In [7]:

print('session.dirty = ', session.dirty)
# => IdentitySet([])

print('session.dirty = ', session.dirty) # => IdentitySet([])

session.dirty =  IdentitySet([])

Use a contextual session¶

(called "db") to grab a separate copy of the batch and verify it has the change.

In [19]:

from sqlmodel import select

with Session(engine) as db:
    batch_copy = db.exec(select(Batch).where(Batch.id == batch.id)).first()
print('batch_copy.description = ', batch_copy.description)
# => foo

from sqlmodel import select with Session(engine) as db: batch_copy = db.exec(select(Batch).where(Batch.id == batch.id)).first() print('batch_copy.description = ', batch_copy.description) # => foo

batch_copy.description =  exceedingly jolly and effortlessly charming and elaborately loquacious

Note that since we grabbed batch_copy using a contextual session, and that context has ended, so has the session, and the object is currently unattached to any session.

In [20]:

print('seesion for batch_copy = ', Session.object_session(batch_copy))
# => None

print('seesion for batch_copy = ', Session.object_session(batch_copy)) # => None

seesion for batch_copy =  None

Now change batch_copy.descripiton,¶

save it with a contextual session, and see how it affected or original Batch instance, which is still alive and attached to the session it was created with.

In [12]:

batch_copy.description += " and effortlessly charming"
with Session(engine) as db:
    db.add(batch_copy)
    db.commit()

batch_copy.description += " and effortlessly charming" with Session(engine) as db: db.add(batch_copy) db.commit()

NOTE: after committing changes to batch_copy using contextual session in this way,¶

trying to access attributes on the object like batch_copy.description raises an error:

Instance <Batch at 0x107b84e10> is not bound to a Session; attribute refresh operation cannot proceed

In [13]:

with Session(engine) as db:
    db.add(batch_copy)
    print('batch_copy.description = ', batch_copy.description)
    # => batch_copy.description =  exceedingly jolly and effortlessly charming

with Session(engine) as db: db.add(batch_copy) print('batch_copy.description = ', batch_copy.description) # => batch_copy.description = exceedingly jolly and effortlessly charming

batch_copy.description =  exceedingly jolly and effortlessly charming

Confirm that our original Batch instance still has the old description¶

In [14]:

print('batch.description before refresh = ', batch.description)
# => batch.description before refresh = exceedingly jolly

print('batch.description before refresh = ', batch.description) # => batch.description before refresh = exceedingly jolly

batch.description before refresh =  exceedingly jolly

Refresh the original Batch instance¶

and check the description again

In [15]:

session.refresh(batch)
print('batch.description after refresh = ', batch.description)
# => batch.description after refresh = exceedingly jolly and effortlessly charming

session.refresh(batch) print('batch.description after refresh = ', batch.description) # => batch.description after refresh = exceedingly jolly and effortlessly charming

batch.description after refresh =  exceedingly jolly and effortlessly charming

Now let's edit batch.description and batch_copy.description¶

and update them both independently to see what happens.

In [16]:

batch.description += " and fantastically flatulent"

batch.description += " and fantastically flatulent"

save the first edit¶

In [17]:

session.commit()

session.commit()

save the second edit¶

In [18]:

with Session(engine) as db:
    batch_copy.description += " and elaborately loquacious"
    db.add(batch_copy)
    db.commit()
    print('batch_copy.description = ', batch_copy.description)
# => batch_copy.description =  exceedingly jolly and effortlessly charming and elaborately loquacious

with Session(engine) as db: batch_copy.description += " and elaborately loquacious" db.add(batch_copy) db.commit() print('batch_copy.description = ', batch_copy.description) # => batch_copy.description = exceedingly jolly and effortlessly charming and elaborately loquacious

batch_copy.description =  exceedingly jolly and effortlessly charming and elaborately loquacious

Unsurprisingly, the 2nd save wins.¶

The changes saved in original Batch instance with it's original session get overwritten by this one.