First pass conversion of NAV Patterns

content/NAVPatterns/singleton-table/index.md

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+title = "singleton-table.md"
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+## Singleton Table
+
+_By Elly Nkya at Microsoft Development Center Copenhagen_
+
+## [![ ][image0]][anchor0]
+
+_  
+_
+
+**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][anchor1].
+
+The **Singleton Table** has two established applications in Dynamics NAV:
+
+1. [**Setup Tables**][anchor2] -- 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:
+
+[watch?v=aQPu s9FkYI&list=PLhZ3P LY7CqmVszuvtJLujFyHpsVN0Uw&index=13][anchor3]
+
+
+
+[anchor0]: 5554.Singleton-Table.png
+[anchor1]: https://en.wikipedia.org/wiki/Singleton_pattern
+[anchor2]: /nav/w/designpatterns/76.setup-table
+[anchor3]: https://www.youtube.com/watch?v=aQPu-s9FkYI&list=PLhZ3P-LY7CqmVszuvtJLujFyHpsVN0U_w&index=13
+
+
+[image0]: 5554.Singleton-Table.png