Postgres distinct on example (Exposed/issues/500)

rest-api/src/main/kotlin/com/example/api/tweeter/ApiController.kt

@@ -10,6 +10,8 @@ import com.example.api.tweeter.search.TweeterSearchHandler
 import com.example.api.tweeter.search.TweeterSearchRequest
 import com.example.api.tweeter.search.TweeterSearchResponse
 import com.example.config.Jackson
+import com.example.util.exposed.functions.postgres.distinctOn
+import com.example.util.exposed.query.toSQL
 import com.fasterxml.jackson.databind.JsonNode
 import io.burt.jmespath.Expression
 import io.burt.jmespath.JmesPath
@@ -19,11 +21,13 @@ import io.swagger.annotations.ApiResponse
 import io.swagger.annotations.ApiResponses
 import mu.KLogging
 import org.funktionale.option.Option
+import org.jetbrains.exposed.sql.*
+import org.jetbrains.exposed.sql.SqlExpressionBuilder.greaterEq
+import org.springframework.transaction.annotation.Transactional
 import org.springframework.web.bind.annotation.*
 import java.time.Instant
 import java.util.*
 
-
 @RestController
 class TweeterApiController(
         private val repo: TweetsRepo,
@@ -37,6 +41,33 @@ class TweeterApiController(
     fun getOne(@PathVariable id: UUID): TweetDto =
             repo[id].toTweetsDto()
 
+    @GetMapping("/api/tweeter/distinctOn")
+    @Transactional(readOnly = true)
+    fun getDistinct(): List<TweetDto> {
+        val selectDistinctOn: CustomFunction<Boolean?> = distinctOn(
+                TweetsTable.message,
+                TweetsTable.comment
+        )
+        val selectColumns: List<Column<*>> = (TweetsTable.columns)
+        val selectWhere: Op<Boolean> = (
+                TweetsTable.createdAt.greaterEq(Instant.EPOCH)
+                )
+
+        val query: Query = TweetsTable
+                .slice(
+                        selectDistinctOn,
+                        *selectColumns.toTypedArray()
+                )
+                .select { selectWhere }
+
+        val SQL: String = query.toSQL()
+        logger.info { "==== Select Distinct On Example === SQL: $SQL " }
+        val dtos: List<TweetDto> = query
+                .map { with(TweetsTable) { it.toTweetsRecord() } }
+                .map { it.toTweetsDto() }
+        return dtos
+    }
+
     @PutMapping("/api/tweeter")
     fun createOne(@RequestBody req: CreateTweetRequest): TweetDto =
             req.toRecord(id = UUID.randomUUID(), now = Instant.now())

rest-api/src/main/kotlin/com/example/util/exposed/functions/common/CustomBooleanFunction.kt

@@ -0,0 +1,21 @@
+package com.example.util.exposed.functions.common
+
+import org.jetbrains.exposed.sql.BooleanColumnType
+import org.jetbrains.exposed.sql.CustomFunction
+import org.jetbrains.exposed.sql.Expression
+import org.jetbrains.exposed.sql.QueryBuilder
+
+/**
+ * Boolean Function (with optional postfix)
+ */
+fun CustomBooleanFunction(
+        functionName: String, postfix: String = "", vararg params: Expression<*>
+): CustomFunction<Boolean?> =
+        object : CustomFunction<Boolean?>(functionName, BooleanColumnType(), *params) {
+            override fun toQueryBuilder(queryBuilder: QueryBuilder) {
+                super.toQueryBuilder(queryBuilder)
+                if (postfix.isNotEmpty()) {
+                    queryBuilder.append(postfix)
+                }
+            }
+        }

rest-api/src/main/kotlin/com/example/util/exposed/functions/postgres/DistinctOn.kt

@@ -0,0 +1,15 @@
+package com.example.util.exposed.functions.postgres
+
+import com.example.util.exposed.functions.common.CustomBooleanFunction
+import org.jetbrains.exposed.sql.CustomFunction
+import org.jetbrains.exposed.sql.Expression
+
+/**
+ * SELECT DISTINCT ON (a,b,c) TRUE, a,b,x,y,z FROM table WHERE ...
+ * see: https://github.com/JetBrains/Exposed/issues/500
+ */
+fun distinctOn(vararg expressions: Expression<*>): CustomFunction<Boolean?> = CustomBooleanFunction(
+        functionName = "DISTINCT ON",
+        postfix = " TRUE",
+        params = *expressions
+)

rest-api/src/main/kotlin/com/example/util/exposed/query/QueryExt.kt

@@ -0,0 +1,6 @@
+package com.example.util.exposed.query
+
+import org.jetbrains.exposed.sql.Query
+import org.jetbrains.exposed.sql.QueryBuilder
+
+fun Query.toSQL():String = prepareSQL(QueryBuilder(false))