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.

This RFC suggest enforcing sound rules for data types and saving lots of storage - it's not clever to keep underlying i.e. 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.

Motivation

There are two possible approaches to this:

1. PropertyValueConstraintDictionary - a Constraint 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"
...

This one is bit harder to implement. That's because present implementation of DictionaryElement (building block of DictionaryElementCollection, which, I guess, ressembles to generic FDO Dictionary) has following definition:

OSGeo::FDO:Common:DictionaryElement (System::String *name, System::String *value) 

This is a bit tricky in the light of PropertyValueConstraintDictionary since dictionary key is a String, while PropertyValue may be of any (reasonable) FDO type for that matter. I guess it would take few additional generic dictionary elements with different Key types (Short... Int16, Int32... Double... etc). Term "name" used for DictionaryElement parameter is not appropriate - term "key" is somehow more suitable. It seems that DictionaryElement and DictionaryElementCollection classes may undergo major reconstruction for PropertyValueConstraintDictionary to work.

2. PropertyValueConstraintLookup - a Constraint with it's definition based on data persisted in another class. Please see related proposed solution and implementation details below.

Proposed Solution

Suggestion is to create new class named PropertyValueConstraintLookup (or PropertyValueConstraintPickList, as Barbara proposed) - a Constraint with it's definition based on data persisted in another class. Such Constraint would only reference another lookup class' contents - it's internal Dictionary would act as a proxy, meaning that no actual data would get associated with such Constraint.

Proposed class fits into OSGeo::FDO::Schema namespace, where PropertyValueConstraintLookup inherits PropertyValueConstraint just like PropertyValueConstraintList and PropertyValueConstraintRange do.

Functionality overview:

1. new class PropertyValueConstraintLookup (or PropertyValueConstraintPickList)
2. takes feature schema name as parameter
3. takes a source class/feature class name from 2) as parameter
4. takes Key property name from 3) for key data source
5. takes Value property name from 3) for value data source (can be same as 4) for that matter)
6. has a getter method returning... well... instantiated FdoReader instead of a dictionary (?)
7. has GroupBy and SortBy optional properties being cast upon 3)
8. Throws an error if there're duplicate keys in 4) - acting as some sort of proxy dictionary

There's also one tricky thing about such class - it's "persistancy" within feature store when underlying data changes (4,5). In other words - it doesn't represent a true constraint. Otherwise, it would probably make it all too complex and resource-consuming to implement. For starters, let's have a consumer developer keep track of underlying data versioning and consistency.

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