如何在DDL脚本中使用TaggedValue及在模板中访问表的TaggedValue？

Hey there! Let's tackle your questions since you're already hands-on with modifying DDL templates—this should align perfectly with your current workflow.

1. General Approach to Using TaggedValues in DDL Scripts #

TaggedValues are your go-to for attaching custom metadata to model elements (like tables, columns, or constraints) that isn't part of standard ER/UML models. To leverage them in DDL, the process boils down to two core steps:

• Add the TaggedValue to your model elements: For example, create a tag like IsTemporalTable for tables, setting values like True or False to flag whether temporal support is needed.
• Modify your DDL template to access the tag: Use conditional logic in the template to generate different SQL code based on the TaggedValue's value.

2. Accessing Table TaggedValues in Your DDL Template for Temporal Tables #

Since you're already tweaking the default DDL template, I'll assume you're using Enterprise Architect (the most common tool for this kind of customization)—here's how to implement the temporal table logic:

Step 1: Add the TaggedValue to Your Tables #

In EA:

• Right-click the target table > Properties > Navigate to the Tagged Values tab.
• Click Add to create a new tag, name it IsTemporalTable, and set its value to True for tables needing temporal support, or False (leave blank if you want a non-temporal default).

Step 2: Modify the DDL Template #

Open the template editor via Settings > Code Generation > Templates, then locate the section where tables are created (usually labeled something like Table Creation). Here's a modified example using SQL Server's temporal table syntax:

CREATE TABLE %TableName% (
    %ListColumns%
    -- Add temporal system columns only if the tag is enabled
    %if TaggedValue("IsTemporalTable") == "True" %
        SysStartTime DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
        SysEndTime DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
        PERIOD FOR SYSTEM_TIME (SysStartTime, SysEndTime)
    %endIf%
)
-- Enable system versioning for tagged tables
%if TaggedValue("IsTemporalTable") == "True" %
    WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.%TableName%_History))
%endIf%;

Key Tips for Success: #

• Match Tag Names Exactly: While EA is case-insensitive for tag names, sticking to consistent spelling (e.g., IsTemporalTable not istemporaltable) avoids unexpected issues.
• Adjust for Your Database: Temporal table syntax varies:
  • MySQL uses WITH SYSTEM VERSIONING directly in the CREATE TABLE block.
  • Oracle uses Flashback Data Archive, requiring ALTER TABLE %TableName% ARCHIVE after creation.
    Tweak the template code to fit your target database's requirements.
• Fallback for Missing Tags: If you want tables without the IsTemporalTable tag to default to non-temporal, adjust the condition to check for tag existence first:

  %if TaggedValueExists("IsTemporalTable") AND TaggedValue("IsTemporalTable") == "True" %
  

Test the Template #

Generate DDL for two tables—one with IsTemporalTable=True and one without—to confirm that temporal table code is only added where intended.

内容的提问来源于stack exchange，提问作者Ruudjah