You're experiencing exactly what Michael pointed you to. Person has a relationship+backref to SourceDatabase though `Person.source_database`. When you create a `John` and assign the existing source_database, sqlalchemy is implicitly adding the Person so that the update you explicitly defined will persist on a commit. In order to disable this, you need to read the section on Cascades that he pointed you to. Specifically look at the section on backrefs.
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