Singleton Table

Singleton Table

By Elly Nkya at Microsoft Development Center Copenhagen

Problem: The developer needs to define a single record that can contain a set of rules and behavior (optional, mandatory, or defaulting mechanisms), that apply to a functionality, and can be configured by a user.

Forces

• You want a central place to define the address and logo of your company (see Company Information table).
• You want to define the no. series that should be used for your sales documents (see Sales & Receivables Setup table).
• You want to know whether your sales documents should be archived (see Sales & Receivables Setup table).
• You want to define the rounding accuracy your system should (see General Ledger Setup table).

Solution: Define a single record that can contain a set of rules and behavior (optional, mandatory, or defaulting mechanisms), that apply to a functionality, and can be configured by a user.

In a functionality that is large enough (such as sales, inventory, fixed) you may want to define a global set of rules, that are configurable by the user.

Implementation

1. Define: Create a Setup Table with Dummy a Primary Key. Typically with type Code=10. Then add fields to define the global rules.

2. Instantiate: Place the instantiation code in a central place where it is guaranteed to be invoked before the functionality uses it. This is done in Codeunit 2.

3. Enforce: Give the user access to the record so that he can change the default setup, by creating a Card page. On the page, enforce the singleton to prevent deletion of the record or insertion of a new record

4. Use: Access the rule in code and use it

NAV Usages

Rounding rules for Unit-Amounts and Amounts are implemented using the Singleton pattern.

1. Define: The General Ledger Setup is used for this.

2. Instantiate: In codeunit 2, the following code is invoked

WITH GLSetup DO
    IF NOT FINDFIRST THEN BEGIN
        INIT;
        INSERT;
    END;

3. Enforce: On the General Ledger Setup. The following properties are setup:

    DeleteAllowed=false,
    InsertAllowed=false

4. Use: Access the rounding rules are used

...
GLSetup.GET;
UnitCostCurrency := ROUND(...,GLSetup."Unit-Amount Rounding Precision"); 
... 

Or if accessing the rule multiple times and performance is a consideration, use lazy instantiation:

...
GetGLSetup;
UnitCostCurrency := ROUND(...,GLSetup."Unit-Amount Rounding Precision");
...

LOCAL GetGLSetup()
IF NOT GLSetupRead THEN
    GLSetup.GET;
GLSetupRead := TRUE;

Related topics

Singleton design pattern.

The Singleton Table has two established applications in Dynamics NAV:

1. Setup Tables – which are commonly storing user setup data in NAV,
2. Cue Tables – used to calculate values for the visual representation of Cues on the NAV role center pages.

YouTube Video of NAV Singleton: