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In some cases, a sequence of several operations must be performed as a single operation: interrupting the sequence of such operations halfway would leave the data in an invalid state. While the sequence is in progress, other processes accessing the database will not see the partial results until the transaction is complete. The sequence make include data queries and manipulations.

In such cases, the sequence of operations may be enclosed in a transaction.

Transactions are formed using the transaction property of the connection object. The connection object may be obtained from any table object. The transaction property can then be used as a context manager in Python’s with statement.

For example, the following code inserts matching entries for the master table Session and its part table Session.Experimenter.

# get the connection object
connection = Session.connection

# insert Session and Session.Experimenter entries in a transaction
with connection.transaction:
    key = {'subject_id': animal_id, 'session_time': session_time}
    Session.insert1({**key, 'brain_region':region, 'cortical_layer':layer})
    Session.Experimenter.insert1({**key, 'experimenter': username})

Here, to external observers, both inserts will take effect together upon exiting from the with block or will not have any effect at all. For example, if the second insert fails due to an error, the first insert will be rolled back.

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