View source for Documentation:Added tagged values to attributes based on the class' tagged values

You can use this EAL expression to mark the attribute that represents each class's configured primary-key column with a <code>PrimaryKey=true</code> tagged value.

== What this expression does ==
This expression reads primary-key metadata from a [[Documentation:TaggedValues|tagged value]] on each class and writes a tagged value to the matching [[Documentation:Attribute|attribute]]. It is intended for models that store database-column metadata in tags:

{| class="wikitable"
! Model element
! Required tagged value
! Meaning
|-
| Class
| <code>Eco.PrimaryKey</code>
| The name of the primary-key column for that class.
|-
| Attribute on that class
| <code>Eco.ColumnName</code>
| The database-column name represented by the attribute.
|-
| Matching attribute
| <code>PrimaryKey=true</code>
| The tag added by this expression.
|}

For example, assume the <code>Customer</code> class has <code>Eco.PrimaryKey</code> with value <code>CustomerId</code>. Its <code>Id</code> attribute has <code>Eco.ColumnName</code> with value <code>CustomerId</code>. Running the expression adds <code>PrimaryKey=true</code> to <code>Customer.Id</code>.

== Prerequisites ==
Before you run the expression:

# Add <code>Eco.PrimaryKey</code> to every class you want to process. Set its value to the relevant column name.
# Add <code>Eco.ColumnName</code> to the attribute that represents that column. Its value must exactly equal the class's <code>Eco.PrimaryKey</code> value.
# Ensure the tagged values are available for the relevant model-element types. The tagged-value editor can refresh its type-sensitive help from [[Documentation:MDrivenDesignerTaggedValueHints|MDrivenDesignerTaggedValueHints]]; see [[Documentation:Tagged values in the Designer|Tagged values in the Designer]].

Tag values are strings. In the example above, <code>CustomerId</code> must have the same spelling and casing in both tags.

== Corrected EAL expression ==
The following EAL expression processes all <code>Eco.PrimaryKey</code> tags, finds matching attributes, and adds <code>PrimaryKey=true</code> only when that attribute does not already have a <code>PrimaryKey</code> tag.

<syntaxhighlight lang="eal">
TaggedValue.allInstances->
 select(tv|tv.Tag='Eco.PrimaryKey')->
 collect(tv | tv.ModelElement->safeCast(Class).Feature->filterOnType(Attribute)->select(a|a.TaggedValue->select(atv|(atv.Tag='Eco.ColumnName') and (atv.Value=tv.Value) )->notEmpty)->
     collect(pka | 
       if pka.TaggedValue->select(atv| atv.Tag='PrimaryKey')->isEmpty then
         let pktv = TaggedValue.Create in
         (
           pktv.Tag := 'PrimaryKey';
           pktv.Value := 'true';
           pka.TaggedValue.add(pktv)
         );
         true
       else
         false
       endif
     )
)
</syntaxhighlight>

=== Why the condition matters ===
The original condition tested for the absence of <code>Eco.ColumnName</code>:

<syntaxhighlight lang="eal">pka.TaggedValue->select(atv| atv.Tag='Eco.ColumnName')->isEmpty</syntaxhighlight>

That condition conflicts with the earlier selection, which already requires <code>pka</code> to have a matching <code>Eco.ColumnName</code>. It therefore evaluates to false for every selected attribute. The corrected condition checks for the absence of <code>PrimaryKey</code> instead:

<syntaxhighlight lang="eal">pka.TaggedValue->select(atv| atv.Tag='PrimaryKey')->isEmpty</syntaxhighlight>

This also makes the expression idempotent: running it again does not add a second <code>PrimaryKey</code> tag to an attribute that was already marked.

== How the expression works ==
# <code>TaggedValue.allInstances</code> starts with every tagged value in the model.
# The first <code>select</code> keeps only class tags named <code>Eco.PrimaryKey</code>.
# <code>tv.ModelElement->safeCast(Class)</code> obtains the class that owns each primary-key tag.
# <code>Feature->filterOnType(Attribute)</code> obtains that class's attributes and excludes other features.
# The nested <code>select</code> keeps attributes whose <code>Eco.ColumnName</code> value equals the class's <code>Eco.PrimaryKey</code> value.
# For each match, the <code>if</code> checks whether a <code>PrimaryKey</code> tag already exists. If it does not, <code>TaggedValue.Create</code> creates the tag, sets <code>Tag</code> to <code>PrimaryKey</code> and <code>Value</code> to <code>true</code>, then adds it to the attribute.

== Expected result and limits ==
The expression changes only attributes that meet both matching conditions. It does not add missing <code>Eco.ColumnName</code> tags, infer a primary key from an attribute name, or remove <code>PrimaryKey</code> tags that no longer match the class metadata.

If a class has no <code>Eco.PrimaryKey</code> tag, or no attribute whose <code>Eco.ColumnName</code> value matches it, that class is unchanged. If more than one attribute has the same matching <code>Eco.ColumnName</code>, each matching attribute is marked.

For general guidance on defining and using your own tag extensions, see [[Documentation:Catching more information in your model]]. For runtime reads of class and feature tag values, see [[Documentation:OCLOperators taggedValue|taggedValue]] and [[Documentation:OCLOperators taggedValueOnFeature|taggedValueOnFeature]].

== See also ==
* [[Documentation:TaggedValues]]
* [[Documentation:Tagged values in the Designer]]
* [[Documentation:Attribute]]
* [[Documentation:Catching more information in your model]]
* [[Documentation:OCLOperators taggedValueOnFeature]]

[[Category:Advanced-Friendly]]
[[Category:Diagrams and Documentation]]
[[Category:Priority 70]]
[[Category:Tagged values, notes, and in model documentation]]
[[Category:Designer]]