ungroup

Ruby

rel.ungroup([:attr])
rel.ungroup([:attr1, :attr2])

TypeScript

rel.ungroup('attr')

Problem

Flatten a relation-valued attribute back into regular tuples.

Example: I have suppliers with nested supplies, and I want to flatten them back into individual rows.

Description

The ungroup operator is the inverse of group. It takes a relation that has a relation-valued attribute (RVA) and expands it, creating one tuple for each combination of the outer tuple and the tuples in the nested relation.

If the nested relation is empty, the outer tuple is removed from the result (similar to an inner join).

Ruby

Multiple attributes can be ungrouped in a single operation by passing an array of attribute names.

TypeScript

The attribute name to ungroup is passed as a string.

Requirements

The specified attributes must be relation-valued attributes in the input relation.

Examples

Basic ungroup

Ruby

Starting with a grouped relation:

# First, create a grouped relation
grouped = supplies.group([:pid, :qty], :supplied_parts)
grouped.to_a

=>
[{:sid=>"S1", :supplied_parts=>#<Bmg::Relation [{:pid=>"P1", :qty=>300}, {:pid=>"P2", :qty=>200}, ...]>},
 {:sid=>"S2", :supplied_parts=>#<Bmg::Relation [{:pid=>"P1", :qty=>300}, {:pid=>"P2", :qty=>400}]>},
 {:sid=>"S3", :supplied_parts=>#<Bmg::Relation [{:pid=>"P2", :qty=>200}]>},
 {:sid=>"S4", :supplied_parts=>#<Bmg::Relation [{:pid=>"P2", :qty=>200}, ...]>}]

# Now ungroup to flatten back
grouped.ungroup([:supplied_parts]).to_a

=>
[{:sid=>"S1", :pid=>"P1", :qty=>300},
 {:sid=>"S1", :pid=>"P2", :qty=>200},
 {:sid=>"S1", :pid=>"P3", :qty=>400},
 {:sid=>"S1", :pid=>"P4", :qty=>200},
 {:sid=>"S1", :pid=>"P5", :qty=>100},
 {:sid=>"S1", :pid=>"P6", :qty=>100},
 {:sid=>"S2", :pid=>"P1", :qty=>300},
 {:sid=>"S2", :pid=>"P2", :qty=>400},
 {:sid=>"S3", :pid=>"P2", :qty=>200},
 {:sid=>"S4", :pid=>"P2", :qty=>200},
 {:sid=>"S4", :pid=>"P4", :qty=>300},
 {:sid=>"S4", :pid=>"P5", :qty=>400}]

TypeScript

Starting with a grouped relation:

// First, create a grouped relation
const grouped = supplies.group(['pid', 'qty'], 'supplied_parts')

// Now ungroup to flatten back
grouped.ungroup('supplied_parts').toArray()

// =>
// [{ sid: "S1", pid: "P1", qty: 300 },
//  { sid: "S1", pid: "P2", qty: 200 },
//  { sid: "S1", pid: "P3", qty: 400 },
//  { sid: "S1", pid: "P4", qty: 200 },
//  { sid: "S1", pid: "P5", qty: 100 },
//  { sid: "S1", pid: "P6", qty: 100 },
//  { sid: "S2", pid: "P1", qty: 300 },
//  { sid: "S2", pid: "P2", qty: 400 },
//  { sid: "S3", pid: "P2", qty: 200 },
//  { sid: "S4", pid: "P2", qty: 200 },
//  { sid: "S4", pid: "P4", qty: 300 },
//  { sid: "S4", pid: "P5", qty: 400 }]

Relationship with group

ungroup and group are inverse operations:

Ruby

# Starting with supplies
supplies.to_a
=> [{:sid=>"S1", :pid=>"P1", :qty=>300}, ...]

# Group by sid
grouped = supplies.group([:pid, :qty], :items)

# Ungroup back - returns to original
grouped.ungroup([:items]).to_a
=> [{:sid=>"S1", :pid=>"P1", :qty=>300}, ...]  # Same as original

TypeScript

// Starting with supplies
supplies.toArray()
// => [{ sid: "S1", pid: "P1", qty: 300 }, ...]

// Group by sid
const grouped = supplies.group(['pid', 'qty'], 'items')

// Ungroup back - returns to original
grouped.ungroup('items').toArray()
// => [{ sid: "S1", pid: "P1", qty: 300 }, ...]  // Same as original