101 lines
3.3 KiB
Markdown
101 lines
3.3 KiB
Markdown
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title = "Singleton Table"
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weight = 1110
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## Singleton Table
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_By Elly Nkya at Microsoft Development Center Copenhagen_
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[![ ][image0]][anchor0]
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**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.
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**Forces**
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* You want a central place to define the address and logo of your company (see Company Information table).
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* You want to define the no. series that should be used for your sales documents (see Sales & Receivables Setup table).
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* You want to know whether your sales documents should be archived (see Sales & Receivables Setup table).
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* You want to define the rounding accuracy your system should (see General Ledger Setup table).
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**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.
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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.
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**Implementation**
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**1\. Define:** Create a Setup Table with Dummy a Primary Key. Typically with type Code=10\. Then add fields to define the global rules.
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**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\.
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**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
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**4\. Use:** Access the rule in code and use it
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**NAV Usages**
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Rounding rules for Unit-Amounts and Amounts are implemented using the Singleton pattern.
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**1\. Define:** The General Ledger Setup is used for this.
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**2\. Instantiate:** In codeunit 2, the following code is invoked
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```al
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WITH GLSetup DO
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IF NOT FINDFIRST THEN BEGIN
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INIT;
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INSERT;
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END;
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```
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**3\. Enforce:** On the General Ledger Setup. The following properties are setup:
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```al
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DeleteAllowed=false,
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InsertAllowed=false
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```
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**4\. Use:** Access the rounding rules are used
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```al
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...
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GLSetup.GET;
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UnitCostCurrency := ROUND(...,GLSetup."Unit-Amount Rounding Precision");
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...
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```
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Or if accessing the rule multiple times and performance is a consideration, use lazy instantiation:
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```al
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...
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GetGLSetup;
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UnitCostCurrency := ROUND(...,GLSetup."Unit-Amount Rounding Precision");
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...
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LOCAL GetGLSetup()
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IF NOT GLSetupRead THEN
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GLSetup.GET;
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GLSetupRead := TRUE;
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```
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**Related topics**
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[Singleton design pattern][anchor1].
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The **Singleton Table** has two established applications in Dynamics NAV:
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1. [**Setup Tables**][anchor2] -- which are commonly storing user setup data in NAV,
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2. **Cue Tables** -- used to calculate values for the visual representation of Cues on the NAV role center pages.
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YouTube Video of NAV Singleton:
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{{< youtube aQPu-s9FkYI>}}
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[anchor0]: 5554.Singleton-Table.png
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[anchor1]: https://en.wikipedia.org/wiki/Singleton_pattern
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[anchor2]: /navpatterns/1-patterns/singleton/singleton-table/setup-table/
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[anchor3]: https://www.youtube.com/watch?v=aQPu-s9FkYI&list=PLhZ3P-LY7CqmVszuvtJLujFyHpsVN0U_w&index=13
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[image0]: 5554.Singleton-Table.png
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