Query virtual attributes

[Speedy37]

Goal: being able to answer to:

• give me all persons with a least 3 cats
• give me the latest version of a document for each distinct document theme
• give the number of cats each persons possess and order them by that number.

Solution: virtual attributes

A virtual attribute is a specially named attribute that represent a computed value and can be used as any other attribute in query (for both conditions and scope)

A virtual attribute can only return a single value.

The $count virtual attribute

Gives the number of values hold by an attribute. If the attribute is sigle value, gives 1 if the value exists, 0 otherwise.

Syntax: $count(attribute)

The range selection virtual attribute

For a set of objets, objects are grouped by a set of attributes then for each group ordered inside it.

Syntax:

• $isLast(attribute ordering) / group by attribute: true if it's the last value in the ordered set of objects optionally grouped by a set of attributes
• $isFirst(attribute ordering) / group by attribute: true if it's the first value in the ordered set of objects optionally grouped by a set of attributes

Examples:

• { $instanceOf: "Person", "$count(_cats)": { $gte: 3 } }
• { $instanceOf: "Document", "$isLast(+_date) / _theme": true }
• { scope: ["+$count(_cats)"] }