project

rel.project([:a, :b, ...])

Problem

Keep only some attributes from a relation (this might cause some tuples to become duplicates of others and thus be removed).

Example: What are the different T-shirt variants we stock, if we only consider colors?

Description

Creates a new relation with a heading whose attributes is a subset of that of the original.

To understand the name, think of a three-dimensional object casting a shadow on a two-dimensional plane. The shadow preserves some of the object’s shape. It is a projection of the three-dimensional object. In the same way, the project operation reduces the dimensionality of a relation.

Requirements

The specified attributes must be part of the input relation’s heading.

Examples

Consult the Overview page for the data model used in this example.

suppliers.project([:city]).to_a

=>
[
 {:city=>"London"},
 {:city=>"Paris"},
 {:city=>"Athens"}
]

Generated SQL

SELECT DISTINCT `t1`.`city`
FROM `suppliers` AS 't1'