Acts as SCD (Slowly Changing Dimension)

This gem provides SCD behaviour for ActiveRecord models. The kind of SCD implemented is “Type 2” acording to en.wikipedia.org/wiki/Slowly_changing_dimension

Models that use this plugin must provide an identity column that establishes the identity of an entity that may be representented by multiple records (iterations), each one of them representing the entity for a specific ‘effective’ period of time.

The effective period of an iteration is defined with day-granularity by two integer columns, effective_from and effective_to using YYYYMMDD format. By default effective_from has value 0 and effective_to 99999999; these special values meaning unlimited periods of time (0 represents the ‘start of time’ and 99999999 the ‘end of time’).

SCD models must provide a compute_identity method to compute the identity attribute.

Here we use this terms:

  • Identity: key that identifies an entity of a SCD through time. The tables will have additional surrogate primary keys (id by default) that identify each iteration of the entity. Here we’ll use identity often in a loose sense to refer to the entity which it identifies.

  • Iteration: the different revisions or variations over time that an Identity may go through. Each iteration of an identity has an effective period in which the iteration is the valid representation of the identity. Here this period is specified by start and end dates (so that variations which have any frequency higher than daily cannot be handled by this method)

Note that the current term, when applied to identities, is used to mean the last iteration if it extends indefinitely to the future (effective_to == 99999999). The last iteration may have a effective end date, meaning that it has disappeared at that date, and in that case it would not be current. Also, if iterations exist with effective dates in the future, the current iterations may not be active at the current date. To get the iteration which is active at the current or any other date, the at methods should be used.

TODO

  • Write Tests

  • Write Documentation

  • Require modal_fields or make it optional?

  • Create generator to add identity to a model and generate migration

  • Release gem 1.0.0

  • Test with Rails 3 & 4