Add a CustomOperator (#753)

exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/Function.kt

@@ -23,6 +23,17 @@ open class CustomFunction<T>(val functionName: String, _columnType: IColumnType,
     }
 }
 
+// create a Function corresponding to a SQL binary operator
+open class CustomOperator<T>(val operatorName: String, _columnType: IColumnType, val expr1: Expression<*>, val expr2: Expression<*>) : Function<T>(_columnType) {
+    override fun toQueryBuilder(queryBuilder: QueryBuilder) = queryBuilder {
+        +"("
+        +expr1
+        +" $operatorName "
+        +expr2
+        +")"
+    }
+}
+
 class NextVal(val seq: Sequence) : Function<Int>(IntegerColumnType()) {
     override fun toQueryBuilder(queryBuilder: QueryBuilder) {
         currentDialect.functionProvider.nextVal(seq, queryBuilder)

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

@@ -259,7 +259,21 @@ class FunctionsTests : DatabaseTestsBase() {
             assertEquals("$initialOp OR $initialOp OR $initialOp OR $initialOp", (initialOp or initialOp or initialOp or initialOp).toString())
             assertEquals("$initialOp OR $initialOp OR $initialOp OR $initialOp", (initialOp or (initialOp or initialOp) or initialOp).toString())
             assertEquals("$initialOp OR ($initialOp AND $initialOp) OR $initialOp", (initialOp or (initialOp and initialOp) or initialOp).toString())
+        }
+    }
 
+    @Test
+    fun testCustomOperator() {
+        // implement a + operator using CustomOperator
+        infix fun Expression<*>.plus(operand: Int) =
+                CustomOperator<Int>("+", IntegerColumnType(), this, intParam(operand))
+
+        withCitiesAndUsers { cities, users, userData ->
+            userData
+                    .select { (userData.value plus 15).eq(35) }
+                    .forEach {
+                        assertEquals(it[userData.value], 20)
+                    }
         }
     }
 }