Add custom database mapping hooks

README.rst

@@ -293,6 +293,31 @@ Distinct types backed by a supported type are parsed through their backing
 type and then converted to the distinct type. This also applies when the
 distinct type is nested in an `Option`_ or `seq`.
 
+Custom Database Mappings
+------------------------
+
+A model module can customize conversion from the database's string
+representation by exporting a ``fromDbHook`` overload:
+
+.. code-block:: Nim
+
+   import std/strutils
+
+   type Money* = object
+     cents*: int64
+
+   proc fromDbHook*(_: typedesc[Money], value: string): Money =
+     Money(cents: value.parseBiggestInt)
+
+The overload must take ``typedesc[TargetType]`` and ``string``, and return
+``TargetType``. It must be visible where Fiber ORM's generated query code is
+declared, so export it when the type lives in another module.
+
+Fiber ORM uses a custom mapping before its built-in conversion for a value.
+``Option`` and ``seq`` remain structural: an absent optional value does not
+call the hook, while present optional values and individual sequence values
+do. Exceptions raised by the hook propagate to the caller.
+
 .. [#f1] Note that this implies that all `NULL`-able fields should be typed
          as optional using `Option[fieldType]`. Conversely, any fields with
          non-optional types should also be constrained to be `NOT NULL` in

fiber_orm.nimble

@@ -19,6 +19,7 @@ requires "namespaced_logging >= 2.0.2"
 task unittest, "Runs the unit test suite.":
   exec "nim c -r --path:src tests/test_datetime"
   exec "nim c -r --path:src tests/test_distinct"
+  exec "nim c -r --path:src tests/test_custom_mapping"
 
 task integrationtest, "Runs the PostgreSQL integration test suite.":
   exec "nim c -r --path:src tests/test_datetime_postgres"

src/fiber_orm.nim

@@ -288,6 +288,35 @@
 ## `JsonNode`_      `jsonb`_
 ## ===============  ======================  =================
 ##
+## Distinct types backed by a supported type are parsed through their backing
+## type and then converted to the distinct type. This also applies when the
+## distinct type is nested in an `Option`_ or `seq`.
+##
+## Custom Database Mappings
+## ------------------------
+##
+## A model module can customize conversion from the database's string
+## representation by exporting a ``fromDbHook`` overload:
+##
+## .. code-block:: Nim
+##
+##    import std/strutils
+##
+##    type Money* = object
+##      cents*: int64
+##
+##    proc fromDbHook*(_: typedesc[Money], value: string): Money =
+##      Money(cents: value.parseBiggestInt)
+##
+## The overload must take ``typedesc[TargetType]`` and ``string``, and return
+## ``TargetType``. It must be visible where Fiber ORM's generated query code is
+## declared, so export it when the type lives in another module.
+##
+## Fiber ORM uses a custom mapping before its built-in conversion for a value.
+## ``Option`` and ``seq`` remain structural: an absent optional value does not
+## call the hook, while present optional values and individual sequence values
+## do. Exceptions raised by the hook propagate to the caller.
+##
 ## .. [#f1] Note that this implies that all `NULL`-able fields should be typed
 ##          as optional using `Option[fieldType]`. Conversely, any fields with
 ##          non-optional types should also be constrained to be `NOT NULL` in

src/fiber_orm/util.nim

@@ -9,6 +9,8 @@ import uuids
 import std/nre except toSeq
 
 type
+  NoDbHook = object
+
   PaginationParams* = object
     pageSize*: int
     offset*: int
@@ -23,6 +25,11 @@ type
     placeholders*: seq[string]
     values*: seq[string]
 
+proc fromDbHook[T](_: typedesc[T], _: string): NoDbHook =
+  ## Default overload used to detect that a consumer has not supplied a
+  ## database mapping for `T`.
+  NoDbHook()
+
 const ISO_8601_FORMATS = @[
   "yyyy-MM-dd'T'HH:mm:ssz",
   "yyyy-MM-dd'T'HH:mm:sszzz",
@@ -225,45 +232,40 @@ proc parseDbArray*(val: string): seq[string] =
   if not (parseState == inQuote) and curStr.len > 0:
     result.add(curStr)
 
-func createParseStmt*(t, value: NimNode): NimNode =
+func createParseStmt*(t, value: NimNode): NimNode
-  ## Utility method to create the Nim code required to parse a value coming from
-  ## the a database query. This is used by functions like `rowToModel` to parse
-  ## the dataabase columns into the Nim object fields.
 
-  if t.typeKind == ntyObject:
+template parseDbValueWithHook(
+    targetType,
+    value,
+    automaticParse: untyped): untyped =
+  block:
+    var target: targetType
+    mixin fromDbHook
+    when typeof(fromDbHook(type(target), value)) is targetType:
+      fromDbHook(type(target), value)
+    else:
+      automaticParse
 
-    if t.getTypeInst == Option.getType:
-      var innerType = t.getTypeImpl[2][0] # start at the first RecList
-      # If the value is a non-pointer type, there is another inner RecList
-      if innerType.kind == nnkRecList: innerType = innerType[0]
-      innerType = innerType[1] # now we can take the field type from the first symbol
 
-      let parseStmt = createParseStmt(innerType, value)
+func unsupportedParseStmt(t: NimNode, description: string): NimNode =
+  let message = newLit("Cannot parse column with " & description)
   result = quote do:
-        if `value`.len == 0:  none[`innerType`]()
+    block:
-        else:                 some(`parseStmt`)
+      {.error: `message`.}
+      default(`t`)
 
-    elif t.getType == UUID.getType:
+
+func createAutomaticParseStmt(t, value: NimNode): NimNode =
+  if t.typeKind == ntyObject:
+    if t.getType == UUID.getType:
       result = quote do: parseUUID(`value`)
 
     elif t.getType == DateTime.getType:
       result = quote do: parsePGDatetime(`value`)
 
-    else: error "Cannot parse column with unknown object type: " & $t.getTypeInst
+    else:
-
+      result = unsupportedParseStmt(
-  elif t.typeKind == ntyGenericInst:
+        t, "unknown object type: " & $t.getTypeInst)
-
-    if t.kind == nnkBracketExpr and
-       t.len > 0 and
-       t[0] == Option.getType:
-
-      var innerType = t.getTypeInst[1]
-      let parseStmt = createParseStmt(innerType, value)
-      result = quote do:
-        if `value`.len == 0:  none[`innerType`]()
-        else:                 some(`parseStmt`)
-
-    else: error "Cannot parse column with unknown generic instance type: " & $t.getTypeInst
 
   elif t.typeKind == ntyDistinct:
     let baseType = t.getTypeImpl[0]
@@ -271,19 +273,12 @@ func createParseStmt*(t, value: NimNode): NimNode =
     result = quote do: `t`(`parseStmt`)
 
   elif t.typeKind == ntyRef:
-
     if $t.getTypeInst == "JsonNode":
       result = quote do: parseJson(`value`)
 
     else:
-      error "Cannot parse column with unknown ref type: " & $t.getTypeInst
+      result = unsupportedParseStmt(
-
+        t, "unknown ref type: " & $t.getTypeInst)
-  elif t.typeKind == ntySequence:
-    let innerType = t[1]
-
-    let parseStmts = createParseStmt(innerType, ident("it"))
-
-    result = quote do: parseDbArray(`value`).mapIt(`parseStmts`)
 
   elif t.typeKind == ntyString:
     result = quote do: `value`
@@ -302,7 +297,47 @@ func createParseStmt*(t, value: NimNode): NimNode =
     result = quote do: parseEnum[`innerType`](`value`)
 
   else:
-    error "Cannot parse column with unknown value type: " & $t.typeKind
+    result = unsupportedParseStmt(
+      t, "unknown value type: " & $t.typeKind)
+
+
+func createParseStmt*(t, value: NimNode): NimNode =
+  ## Create code to parse a value returned by a database query.
+  ##
+  ## `Option` and `seq` remain structural mappings owned by Fiber ORM. Their
+  ## contained values are parsed recursively and may use `fromDbHook`.
+  if t.typeKind == ntyObject and t.getTypeInst == Option.getType:
+    var innerType = t.getTypeImpl[2][0] # start at the first RecList
+    # If the value is a non-pointer type, there is another inner RecList.
+    if innerType.kind == nnkRecList: innerType = innerType[0]
+    innerType = innerType[1] # take the field type from the first symbol
+
+    let parseStmt = createParseStmt(innerType, value)
+    result = quote do:
+      if `value`.len == 0:  none[`innerType`]()
+      else:                 some(`parseStmt`)
+    return
+
+  if t.typeKind == ntyGenericInst and
+     t.kind == nnkBracketExpr and
+     t.len > 0 and
+     t[0] == Option.getType:
+    let innerType = t.getTypeInst[1]
+    let parseStmt = createParseStmt(innerType, value)
+    result = quote do:
+      if `value`.len == 0:  none[`innerType`]()
+      else:                 some(`parseStmt`)
+    return
+
+  if t.typeKind == ntySequence:
+    let innerType = t[1]
+    let parseStmt = createParseStmt(innerType, ident("it"))
+    result = quote do: parseDbArray(`value`).mapIt(`parseStmt`)
+    return
+
+  let automaticParseStmt = createAutomaticParseStmt(t, value)
+  result = quote do:
+    parseDbValueWithHook(`t`, `value`, `automaticParseStmt`)
 
 func fields(t: NimNode): seq[tuple[fieldIdent: NimNode, fieldType: NimNode]] =
   #[
@@ -396,7 +431,6 @@ macro rowToModel*(modelType: typed, row: seq[string]): untyped =
       fieldIdent,
       createParseStmt(fieldType, itemLookup)))
     idx += 1
-
 #[
 macro listFields*(t: typed): untyped =
   var fields: seq[tuple[n: string, t: string]] = @[]

tests/custom_db_types.nim

@@ -0,0 +1,20 @@
+import std/[strutils]
+
+
+type
+  ValidatedId* = distinct string
+
+  Money* = object
+    cents*: int64
+
+
+proc fromDbHook*(
+    _: typedesc[ValidatedId],
+    value: string): ValidatedId =
+  if not value.startsWith("id_"):
+    raise newException(ValueError, "validated ID must begin with id_")
+  ValidatedId(value.toUpperAscii)
+
+
+proc fromDbHook*(_: typedesc[Money], value: string): Money =
+  Money(cents: value.parseBiggestInt)

tests/test_custom_mapping.nim

@@ -0,0 +1,45 @@
+import std/[options, unittest]
+
+import fiber_orm/util
+
+import ./custom_db_types
+
+
+type CustomModel = object
+  id: ValidatedId
+  optionalId: Option[ValidatedId]
+  absentId: Option[ValidatedId]
+  relatedIds: seq[ValidatedId]
+  balance: Money
+
+
+suite "custom database mappings":
+
+  test "uses custom mappings for values nested in structural types":
+    let model = rowToModel(CustomModel, @[
+      "id_primary",
+      "id_optional",
+      "",
+      "{id_first,id_second}",
+      "1250",
+    ])
+
+    check:
+      string(model.id) == "ID_PRIMARY"
+      model.optionalId.isSome
+      string(model.optionalId.get) == "ID_OPTIONAL"
+      model.absentId.isNone
+      model.relatedIds.len == 2
+      string(model.relatedIds[0]) == "ID_FIRST"
+      string(model.relatedIds[1]) == "ID_SECOND"
+      model.balance.cents == 1_250
+
+  test "propagates errors from custom mappings without falling back":
+    expect ValueError:
+      discard rowToModel(CustomModel, @[
+        "invalid",
+        "",
+        "",
+        "{}",
+        "1250",
+      ])