feat: Return non-nullable column def in addIsNotNull (#111)

Author: jwoliver-faire

docs/nullability.md

@@ -128,3 +128,27 @@ But for now you can do:
     // [in a projection]
     aNullableString = TypedProjections.`null`(),
 ```
+
+## `addIsNotNull` returns a non-nullable column
+
+The `addIsNotNull` method not only adds an `IS NOT NULL` criterion to the query, but also returns a non-nullable version of the column. This is useful when
+you want to use the column in a projection after filtering out nulls — the returned column has the nullability stripped from its type parameter.
+
+```kotlin
+session.project(BookTable) { books ->
+    // books.notes is ColumnDef<String?>, but after addIsNotNull it becomes ColumnDef<String>
+    val notes = addIsNotNull(books.notes)
+    project(YawnProjections.pair(books.name, notes))
+}
+```
+
+This works with both regular columns and foreign key join columns:
+
+```kotlin
+session.project(BookTable) { books ->
+    // books.publisher is JoinColumnDefWithForeignKey<*, *, Publisher?>
+    // publisherFk is ColumnDef<Long> (non-nullable FK)
+    val publisherFk = addIsNotNull(books.publisher)
+    project(YawnProjections.pair(books.name, publisherFk))
+}
+```

yawn-api/src/main/kotlin/com/faire/yawn/criteria/query/TypeSafeCriteriaWithWhere.kt

@@ -293,14 +293,16 @@ sealed interface TypeSafeCriteriaWithWhere<SOURCE : Any, T : Any> {
 
     fun <F> addIsNotNull(
         column: YawnTableDef<SOURCE, *>.JoinColumnDefWithForeignKey<*, *, F>,
-    ) {
-        addIsNotNull(column.foreignKey)
+    ): YawnDef<SOURCE, *>.YawnColumnDef<F & Any> {
+        return addIsNotNull(column.foreignKey)
     }
 
     fun <F> addIsNotNull(
         column: YawnDef<SOURCE, *>.YawnColumnDef<F>,
-    ) {
+    ): YawnDef<SOURCE, *>.YawnColumnDef<F & Any> {
         add(YawnRestrictions.isNotNull(column))
+        @Suppress("UNCHECKED_CAST")
+        return column as YawnDef<SOURCE, *>.YawnColumnDef<F & Any>
     }
 
     fun <F> addIsNull(

yawn-database-test/src/test/kotlin/com/faire/yawn/database/YawnCriterionTest.kt

@@ -1,5 +1,6 @@
 package com.faire.yawn.database
 
+import com.faire.yawn.project.YawnProjections
 import com.faire.yawn.query.YawnRestrictions.and
 import com.faire.yawn.query.YawnRestrictions.between
 import com.faire.yawn.query.YawnRestrictions.eq
@@ -645,6 +646,38 @@ internal class YawnCriterionTest : BaseYawnDatabaseTest() {
         }
     }
 
+    @Test
+    fun `addIsNotNull filters to non-null rows`() {
+        transactor.open { session ->
+            val results = session.project(BookTable) { books ->
+                val notes = addIsNotNull(books.notes)
+                addLike(notes, "Note for The Hobbit and Harry Potter")
+                orderAsc(books.name)
+                project(books.name)
+            }.list()
+
+            assertThat(results).containsExactly("Harry Potter", "The Hobbit")
+        }
+    }
+
+    @Test
+    fun `addIsNotNull with FK column filters to non-null rows`() {
+        transactor.open { session ->
+            val publisherIdMap = session.project(PublisherTable) { publishers ->
+                project(YawnProjections.pair(publishers.name, publishers.id))
+            }.set().toMap()
+
+            val results = session.project(BookTable) { books ->
+                val publisherFk = addIsNotNull(books.publisher)
+                addEq(publisherFk, publisherIdMap.getValue("Penguin"))
+                orderAsc(books.name)
+                project(books.name)
+            }.list()
+
+            assertThat(results).containsExactly("Harry Potter", "The Emperor's New Clothes")
+        }
+    }
+
     @Test
     fun `addAndOfNotNull filters out null criteria`() {
         transactor.open { session ->

yawn-database-test/src/test/kotlin/com/faire/yawn/database/YawnProjectionTest.kt

@@ -773,9 +773,59 @@ internal class YawnProjectionTest : BaseYawnDatabaseTest() {
         }
     }
 
+    @Test
+    fun `project non-null column with addIsNotNull`() {
+        transactor.open { session ->
+            // books.notes is String? — addIsNotNull filters to non-null rows and returns ColumnDef<String>
+            val results = session.project(BookTable) { books ->
+                val notes = addIsNotNull(books.notes)
+                orderAsc(books.name)
+                project(YawnProjectionTest_BookNameAndNotesProjection.create(books.name, notes))
+            }.list()
+
+            // Only books with non-null notes: Harry Potter, Lord of the Rings, The Hobbit
+            assertThat(results).containsExactly(
+                BookNameAndNotes(name = "Harry Potter", notes = "Note for The Hobbit and Harry Potter"),
+                BookNameAndNotes(name = "Lord of the Rings", notes = "Note for Lord of the Rings"),
+                BookNameAndNotes(name = "The Hobbit", notes = "Note for The Hobbit and Harry Potter"),
+            )
+        }
+    }
+
+    @Test
+    fun `project non-null FK column with addIsNotNull`() {
+        transactor.open { session ->
+            val publisherIdMap = session.project(PublisherTable) { publishers ->
+                project(YawnProjections.pair(publishers.name, publishers.id))
+            }.set().toMap()
+
+            // books.publisher is Publisher? — addIsNotNull on FK should filter to non-null rows and return non-null
+            // FK column
+            val results = session.project(BookTable) { books ->
+                val publisherFk = addIsNotNull(books.publisher)
+                orderAsc(books.name)
+                project(YawnProjections.pair(books.name, publisherFk))
+            }.list()
+
+            // 4 books have publishers: Harry Potter, Lord of the Rings, The Emperor's New Clothes, The Hobbit
+            assertThat(results).containsExactly(
+                "Harry Potter" to publisherIdMap.getValue("Penguin"),
+                "Lord of the Rings" to publisherIdMap.getValue("HarperCollins"),
+                "The Emperor's New Clothes" to publisherIdMap.getValue("Penguin"),
+                "The Hobbit" to publisherIdMap.getValue("Random House"),
+            )
+        }
+    }
+
     @YawnProjection
     internal data class BookStatistics(
         val totalBooks: Long,
         val totalPages: Long,
     )
+
+    @YawnProjection
+    internal data class BookNameAndNotes(
+        val name: String,
+        val notes: String,
+    )
 }