Describe the inspiration for your proposal

very often data are inserted in a db with constrains like a unique key
key can be one properties or a set of properties

it occurs that we may insert , update or manipulate data in bulk , and that as a pre requisite we have not unicity of the entire object but unicity on the subset of an object

for instance

{
 "type" : "array",
 "uniqueItems" : true, 
 "items": {
 "type" : "object",
 "required" : ["id", "dummy"] 
 "properties" : {
 "id" : { "type" : "integer" },
 "dummy" : { "type" : "integer" }
 }
 }
} 

For instance making this failing base on the subschema : "key"
[ { "id" : 1 , "dummy" : 12 } , {"id" : 1 , "dummy" : 13 }]

Describe the proposal

allows uniqueItems to be either a boolean either a schema like

{
 "type" : "array",
 "uniqueItems" : { "type" : "object" , "properties" : {"id" : { "type" : "integer" } } 
 "items": {
 "type" : "object",
 "required" : ["id", "dummy"] 
 "properties" : {
 "id" : { "type" : "integer" },
 "dummy" : { "type" : "integer" }
 }
 }
} 

Describe alternatives you've considered

an alternative would be a meta data on top of the array to indicate "uniqueKey" as a complement of uniqueItems 👎

like

{
 "type" : "array",
 "uniqueItems" : true,
 "uniqueKey" : ["id"] 
 "items": {
 "type" : "object",
 "required" : ["id", "dummy"] 
 
 "properties" : {
 "id" : { "type" : "integer" },
 "dummy" : { "type" : "integer" }
 }
 }
} 

another alternative would be to leverage a meta data on top of the properties (may be better )

{
 "type" : "array",
 "uniqueItems" : true,
 "items": {
 "type" : "object",
 "required" : ["id", "dummy"] 
 
 "properties" : {
 "id" : { "type" : "integer" , "key": true },
 "dummy" : { "type" : "integer" }
 }
 }
} 

Additional context

my context is unicity toward not a single properties like id , but 'kind of' composite key , spread on several properties