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Introduction

For the wishlist item

Requirements

When executing a batch the new metadata of the item should be able to be determined by

• The previous/current metadata of the item

• Combinations of fields

• Conditions

• Textual operations

Solution

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Introduction

Using Metadata Expressions a new batch task is available and named BatchExpressionTemplatesTask. It takes as an argument a list of metadata sidecar e.g.-like arguments. An example of such metadata sidecar is

{
    "Descriptive": {
       "Title": "Metadata Expression Analysis"
    },#{R('Descriptive.Title').toUpperCase()}",
       "Description": "#{R('Descriptive.Title') + ': ' + R('Dynamic.CustomDescription')}",
     "Dynamic  "Keywords": {"Keyword": "#{R('Dynamic.CustomTag')}"}
    },
    "CustomFieldDynamic": {
 "Some  value"     }
}

This metadata sidecar can now use a strict Spring Expression Language (Spel) subset to perform more complex operations. You can put one or more expressions #{...} in the JSON or XML. Inside an expression, you can reference the metadata using R(<Dotted key>) or record(<Dotted key>) for example R('Descriptive.Title') or record('Descriptive.Title').

Example input

"Orientation": "#{(R('Technical.Width') > R('Technical.Height')) ? 'Horizontal' : 'Vertical'}",
        "AdditionalInformation": "My custom field"
    }
}

Merging

When the templated sidecar is evaluated against a record it will result for example in the following metadata sidecar

The expression for each JSON property must evaluate to one of the following types

{
    "Descriptive": {
       "Title": "R('Descriptive.Title').toUpperCase()MY TEST FILE",
       "Description": "R('Descriptive.Title') + ': ' + R('Dynamic.CustomField')My test file: this is a test file",
       "Keywords": {"Keyword": "{R('Dynamic.CustomField')}"["My custom tag"]}
    },
    "Dynamic": {
        "CustomFieldOrientation": "R('Dynamic.CustomField2') == 'X' ? 'Option A' : 'Option B'"Horizontal",
        "AdditionalInformation": "My custom field"
    }
}

This metadata sidecar will then be merged against the record using the same rules as when MediaHaven Rest API - Updating a record.

Output after evaluating the expression

After evaluating all expressions in the input metadata sidecar, the result is a new metadata sidecar in JSON. The conversion follows the following rules

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All special characters are automatically escaped

The output of the metadata accessor

The output of the metadata accessor R or record is the following depending on which field type the dotted key refers to:

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Level

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Case

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Effect

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Example Expression

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Example Output

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Top

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SimpleField

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String

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R('Descriptive.Title')

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My title

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Top

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ListField

...

List of Strings

...

R('Descriptive.Keywords')

...

["A", "B", "C"]

...

Top

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MapField

...

Map of Strings

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R('Structural.Versioning')

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{"Status": "HEAD", "Version": "2"}

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Top

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MultiItemField

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Map of List of Strings

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R('Descriptive.Authors')

...

{"Authors": ["Cedric"], "Director": ["Alice", "Bob"]}

...

Top

Others

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Not allowed

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Sub

...

Child of ListField

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List of Strings

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R('Descriptive.Keywords.Keyword')

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["A", "B", "C"]

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Sub

...

Child of MultiItemField

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List of Strings

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R('Descriptive.Authors.Director')

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["Alice", "Bob"]

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Sub

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Child of MapField

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String

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R('Structural.Versioning.Status')

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HEAD

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Sub

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Other

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Not allowed

The R takes an optional second argument, which is the default value. When it is not specified the default value is the empty string.

Security

The available features of SPel are described here: https://docs.spring.io/spring-framework/docs/3.2.x/spring-framework-reference/html/expressions.html. We must disable all features except

• Literal expressions

• Boolean and relational operators

• Method invocation

• Inline lists

• Ternary operator

• The method name “R” or “record”

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