test: Add test coverage for column transforms (JetBrains#1687)

* Add coverage for ColumnWithTransform

* formatting tweaks

exposed-tests/src/test/kotlin/org/jetbrains/exposed/sql/tests/shared/entities/ColumnWithTransformTest.kt

@@ -0,0 +1,84 @@
+package org.jetbrains.exposed.sql.tests.shared.entities
+
+import org.jetbrains.exposed.dao.IntEntity
+import org.jetbrains.exposed.dao.IntEntityClass
+import org.jetbrains.exposed.dao.id.EntityID
+import org.jetbrains.exposed.dao.id.IntIdTable
+import org.jetbrains.exposed.sql.Op
+import org.jetbrains.exposed.sql.select
+import org.jetbrains.exposed.sql.tests.DatabaseTestsBase
+import org.jetbrains.exposed.sql.tests.shared.assertEquals
+import org.junit.Test
+
+object TransformationsTable : IntIdTable() {
+    val value = varchar("value", 50)
+}
+
+object NullableTransformationsTable: IntIdTable() {
+    val value = varchar("nullable", 50).nullable()
+}
+
+class TransformationEntity(id: EntityID<Int>) : IntEntity(id) {
+    companion object : IntEntityClass<TransformationEntity>(TransformationsTable)
+    var value by TransformationsTable.value.transform(
+        toColumn = { "transformed-$it" },
+        toReal = { it.replace("transformed-", "") }
+    )
+}
+
+class NullableTransformationEntity(id: EntityID<Int>) : IntEntity(id) {
+    companion object : IntEntityClass<NullableTransformationEntity>(NullableTransformationsTable)
+    var value by NullableTransformationsTable.value.transform(
+        toColumn = { "transformed-$it" },
+        toReal = { it?.replace("transformed-", "") }
+    )
+}
+
+class ColumnWithTransformTest: DatabaseTestsBase() {
+
+    @Test
+    fun `set and get value`() {
+        withTables(TransformationsTable) {
+            val entity = TransformationEntity.new {
+                value = "stuff"
+            }
+
+            assertEquals("stuff", entity.value)
+
+            val row = TransformationsTable.select(Op.TRUE)
+                .first()
+
+            assertEquals("transformed-stuff", row[TransformationsTable.value])
+        }
+    }
+
+    @Test
+    fun `set and get nullable value - while present`() {
+        withTables(NullableTransformationsTable) {
+            val entity = NullableTransformationEntity.new {
+                value = "stuff"
+            }
+
+            assertEquals("stuff", entity.value)
+
+            val row = NullableTransformationsTable.select(Op.TRUE)
+                .first()
+
+            assertEquals("transformed-stuff", row[NullableTransformationsTable.value])
+        }
+    }
+
+    @Test
+    fun `set and get nullable value - while absent`() {
+        withTables(NullableTransformationsTable) {
+            val entity = NullableTransformationEntity.new {}
+
+            assertEquals(null, entity.value)
+
+            val row = NullableTransformationsTable.select(Op.TRUE)
+                .first()
+
+            assertEquals(null, row[NullableTransformationsTable.value])
+        }
+    }
+}