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Calculated/copied attributes to support advanced single-table design in DynamoDB

Project description

PynamoDB Polymorph

This package has utilities that are useful when using PynamoDB with a single-table (polymorphic) design and overloading Global Secondary Indexes.

Take for example a Publisher class that needs to use a compound key of their type prefix and their name. If you rely on callers doing the copy in code there is possibility for them to update the name without also updating the GSI sort key that contains the name. The CompoundTemplateAttribute uses string.Template templating to fill unicode or numeric attributes into a string template to build keys automatically.

class Publisher(Base, discriminator="Publisher"):
    name = UnicodeAttribute()
    description = UnicodeAttribute(default="")
    gsi1_pk = CopiedDiscriminatorAttribute(source="cls")

    _slug = "PUBLISHER"
    gsi1_sk = JoinedUnicodeAttribute(attrs=["_slug", 'name'], sep='#')

    # Alternative way to specify gsi1_sk using a `string.Template`.
    # This is best for cases that aren't simply joining attrs with a character
    # gsi1_sk = CompoundTemplateAttribute(
    #     template="PUBLISHER#$name",
    #     attrs=["name"],
    # )

The template above will take any value in name and fill it in to the template, so name="Random Book Publishing House" will become a gsi1_sk="PUBLISHER#Random Book Publishing House".

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