Tests if an attribute exists in a document.
Command Syntax
attr(attribute).exists()
Usage
const d = new Dynaflo(config);
d.table('Movies')
.update({year: 2015})
.set({title: 'New Title'})
.when(attr('title').exists())
.run()
.then(data => {
// only updates the title when the document has a title attribute
});
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