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The Lookup property model

• Introduction
• Types of lookups
• Custom built lookup lists
• Lookup lists of internal system objects
• The lookup data type architecture
• Archie2Lookup Property type
• Lookup Property
• LookupType
• LookupItem 

Introduction

The lookup data type is designed to allow the user to specify a named property that will present the user with a dropdown list of legal values on the instance create and instance edit screens.

This section describes how the data type works and how it should be used, including the various options available to users of this data type.

Types of lookups

The Lookup data type essentially has two modes of operation: Custom and System

Custom built lookup lists

When using this form of lookup, you have to build your own lists of legal values per lookup set. When EVA Netmodeler first starts up, it retrieves all lookup sets from the database and builds a memory representation for them. This is done for performance reasons.

Obviously, it will be necessary to refresh the memory representation of the lookup structure whenever items are added, deleted or changed in a lookup set, or whenever a lookup set is added, deleted or changed. This is done via a set of event subscriptions, which will be explained later on in this document.

Lookup lists of internal system objects

The architecture of this form of lookup data type usage is simpler in the sense that values are retrieved from the database every time the data type is used, so there is no need to maintain a memory representation of these types of lookups.

This form of usage allows a user to display a list of internal EVA Netmodeler objects as a dropdown box during editing or creation of instances. Typical objects that can be displayed are: Types, Instances of a type, RelTypes, Named Properties, Domains, Users and so forth.

The lookup data type architecture

A lookup property in EVA Netmodeler is a property of which the range of possible values is restricted to one of a defined set of values. The defined set is named and managed as independent entity - the same set can be used to constrain any number of lookup properties. The possible values in the sets are also managed independently.

These concepts are now defined more formally as they are implemented in EVA Netmodeler.

Archie2Lookup Property type

This is the base property type that is used to define all Lookup Properties in EVA Netmodeler.

Lookup Property

This is a named property of the type Archie2Lookup. Lookup properties have user assigned names and can be used as properties on one or more Node Types

LookupType

Items of this type are named lookup sets which are used to populate the Lookup Properties that reference them.

LookupItem

These are the actual items to be included in a named lookup sets.

For each set or lookup values, there will be an instance of LookupType, and for each value in the set, there will be an instance of LookupItem, connected to its set via the "includes" relationship. Refer to the section on Configuring and Using Lookup Property Types for a worked example of defining a custom lookup type.

The names of the types used to store the lookup sets and values are controlled via system settings. Refer to the system settings section for more information on these settings and how to change them.

In addition to the above data definitions, EVA Netmodeler also uses system events to refresh the lookup sets in memory whenever the underlying lookup items change. EVA Netmodeler uses its own event subscription mechanism to achieve this. Refer to the Events section for further information on how events work in EVA Netmodeler.