FDO RFC 15 – FDO PropertyValueConstraintDictionary Class

This page contains a request for comments document (RFC) for the FDO Open Source project. More FDO RFCs can be found on the RFCs page.

Status

RFC Template Version	(1.0)
Submission Date	February 9, 2008
Last Modified	Maksim Sestic Timestamp
Author	Maksim Sestic
RFC Status	Draft
Implementation Status	Not Applicable
Proposed Milestone	3.3.1.0
Assigned PSC guide(s)	(when determined)
Voting History
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Overview

Presently, PropertyValueConstraintList may contain only plain list of predefined values forced upon particular PropertyValue (minus null value). In my opinion, it somehow beats sole purpose of Constraints in a view of overall datastore consolidation. Therefor, suggestion is to create new class named PropertyValueConstraintDictionary (or whatever) that will keep pairs of [object,string] values, where object is a key dictionary member (an item in present PropertyValueConstraintList), while associated string represents localized description of the value itself.

Example:

RoadTypePropertyValueConstraintDictionary:

null , "n/a"
0    , "Highway"
1    , "Main road"
2    , "Street"
...

Therefor enforcing sound "rules" for data types and saving lots of storage - it's not clever to keep underlying RoadTypePropertyValue as String since it takes too much space, it's not possible to change associated meaning of data once the data is in the store, while schema user from different country may not understand "by default" what Main road means, etc.
On the client side, developers may build more robust UIs to deal with FDO-based data. I.e. a ComboBox instead of plain text field, where combo has it's values set to predefined PropertyValueConstraintDictionary object range and displayed members consist of associated string values.
In that way users will be able to pick between "Highway", "Main road" or "Street", while underlying datastore keeps it's short or boolean (or any other) universal representation.

Implementation Details

Proposed class fits into OSGeo::FDO::Schema namespace, where PropertyValueConstraintDictionary inherits PropertyValueConstraint just like PropertyValueConstraintList and PropertyValueConstraintRange do.
It should have a PropertyValueConstraintDictionary() getter/setter, where getter returns Dictionary FDO implementation that holds actual key/value pairs (in this particular case Key type represents underlying PropertyValue type, while Value type is String).

Introduced Pros

• Major storage saver
• Splits data value from data representation (0 = "Highway" or True = "Yes")
• Enables creating of flags via binary math (1+2 = 3 = "Main road" AND "Street")
• Resolves data Localization issues
• Enables more advanced UI solutions on client side