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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def migrate_flow_collection(apps, schema_editor):
 """Migrate 'flow_collection' field to 'entity_type'."""

    Process = apps.get_model('flow', 'Process')
    DescriptorSchema = apps.get_model('flow', 'DescriptorSchema')

    for process in Process.objects.all():
        process.entity_type = process.flow_collection
        process.entity_descriptor_schema = process.flow_collection

        if (process.entity_descriptor_schema is not None and
                not DescriptorSchema.objects.filter(slug=process.entity_descriptor_schema).exists()):
            raise LookupError(
                "Descriptow schema '{}' referenced in 'entity_descriptor_schema' not "
                "found.".format(process.entity_descriptor_schema)
            )

        process.save()