ProjectionExpression

The ProjectionExpression API is common to both the Type Safe High Level API (with the exception the primary keys API) and the Low Level API - however the way one is constructed is different.

High Level API construction

final case class Person(id: String, group: String name: String)
object Person {
  implicit val schema: Schema.CaseClass3[String, String, String, Person] = DeriveSchema.gen[Person]
  
  // id and name are ProjectionExpressions
  val (id, group, name) = ProjectionExpression.accessors[Person]
}

Low Level API construction

// id and name are ProjectionExpressions
val id = ProjectionExpression.$("id")
val name = ProjectionExpression.$("name")

For more details see $ and Parse functions section

Type Safety

The other notable difference is that High Level API is totally type safe and many subtle errors are caught at compile time:

Person.id === 1 // will not compile
Person.id === "1" && Person.name === 2 // will not compile
Person.id === "1" && Employee.name === "2" // will not compile

ProjectionExpression.$("name") === 1 // will compile!

ProjectionExpression API Summary

Once we have a ProjectionExpression we can use it as a springboard to create further expressions such as ConditionExpression, UpdateExpression and PrimaryKeyExpression:

Category	Method	Description
Primary Keys (High Level API only)	partitionKey	Person.id.partitionKey === "1"
	sortKey	Person.id.partitionKey === "1" && Person.group.sortKey === "2"
ConditionExpression / FilterExpression	===	Equality
	<>	Inequality
	>	Greater than
	>=	Greater than or equal
	<	Less than
	<=	Less than or equal
	exists	returns true if the attribute exists
	notExists	returns true if the attribute does not exists
	size	returns size of attribute. Applies to all types except Number and Boolean
	isXXXX	There is a function per type eg isBinary - returns true if the attribute is of the specified type
	between(from, to)	returns true if the attribute is between the supplied values, inclusive
	beginsWith	Only applies to string attributes
	inSet	returns true if the attribute is in the supplied set
	in(a, b, c ...)	returns true if the attribute matches one of the supplied values
	contains	returns true if the attribute contains the supplied value - applies to a String or a Set
UpdateExpression	+	combines update actions eg Person.name.set(42) + Person.age.set(42)
	set	Set an attribute Person.name.set("John")
	setIfNotExists	Set attribute if it does not exists Person.name.setIfNotExists("John")
	appendList	Add supplied list to the end of this list attribute
	prepend	Prepend an element to a list attribute
	prependList	Prepend a list to list attribute
	deleteFromSet	delete all elements that match the supplied set
	add(a: To)	adds this value as a number attribute if it does not exists, else adds the numeric value to the existing attribute
	addSet	Adds this set as an attribute if it does not exists, else if it exists it adds the elements of the set
	remove(index: Int)	remove an element at the specified index
	remove	Removes this path expression from an item