Add multivalued insert context for defaultsreview/mike_bayer/ticket_4075

Added a new method :class:`.DefaultExecutionContext.current_parameters`
which is used within a function-based default value generator in
order to retrieve the current parameters being passed to the statement.
The new function differs from the ``.current_parameters`` attribute in
that it also provides for optional grouping of parameters that
correspond to a multi-valued "insert" construct.  Previously it was not
possible to identify the subset of parameters that were relevant to
the function call.

Change-Id: I6894c7b4a2bce3e83c3ade8af0e5b2f8df37b785
Fixes: #4075

4 files changed, 90 insertions, 4 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index b5c95cb17..1719de516 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -977,7 +977,7 @@ class Connection(Connectable):
         except BaseException as e:
             self._handle_dbapi_exception(e, None, None, None, None)
 
-        ret = ctx._exec_default(default, None)
+        ret = ctx._exec_default(None, default, None)
         if self.should_close_with_result:
             self.close()
 
diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 227ff0845..4b9aa9493 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -1177,10 +1177,11 @@ class DefaultExecutionContext(interfaces.ExecutionContext):
                 self.root_connection._handle_dbapi_exception(
                     e, None, None, None, self)
 
-    def _exec_default(self, default, type_):
+    def _exec_default(self, column, default, type_):
         if default.is_sequence:
             return self.fire_sequence(default, type_)
         elif default.is_callable:
+            self.current_column = column
             return default.arg(self)
         elif default.is_clause_element:
             # TODO: expensive branching here should be
@@ -1195,17 +1196,97 @@ class DefaultExecutionContext(interfaces.ExecutionContext):
         else:
             return default.arg
 
+    current_parameters = None
+    """A dictionary of parameters applied to the current row.
+
+    This attribute is only available in the context of a user-defined default
+    generation function, e.g. as described at :ref:`context_default_functions`.
+    It consists of a dictionary which includes entries for each column/value
+    pair that is to be part of the INSERT or UPDATE statement. The keys of the
+    dictionary will be the key value of each :class:`.Column`, which is usually
+    synonymous with the name.
+
+    Note that the :attr:`.DefaultExecutionContext.current_parameters` attribute
+    does not accommodate for the "multi-values" feature of the
+    :meth:`.Insert.values` method.  The
+    :meth:`.DefaultExecutionContext.get_current_parameters` method should be
+    preferred.
+
+    .. seealso::
+
+        :meth:`.DefaultExecutionContext.get_current_parameters`
+
+        :ref:`context_default_functions`
+
+    """
+
+    def get_current_parameters(self, isolate_multiinsert_groups=True):
+        """Return a dictionary of parameters applied to the current row.
+
+        This method can only be used in the context of a user-defined default
+        generation function, e.g. as described at
+        :ref:`context_default_functions`. When invoked, a dictionary is
+        returned which includes entries for each column/value pair that is part
+        of the INSERT or UPDATE statement. The keys of the dictionary will be
+        the key value of each :class:`.Column`, which is usually synonymous
+        with the name.
+
+        :param isolate_multiinsert_groups=True: indicates that multi-valued
+         INSERT contructs created using :meth:`.Insert.values` should be
+         handled by returning only the subset of parameters that are local
+         to the current column default invocation.   When ``False``, the
+         raw parameters of the statement are returned including the
+         naming convention used in the case of multi-valued INSERT.
+
+        .. versionadded:: 1.2  added
+           :meth:`.DefaultExecutionContext.get_current_parameters`
+           which provides more functionality over the existing
+           :attr:`.DefaultExecutionContext.current_parameters`
+           attribute.
+
+        .. seealso::
+
+            :attr:`.DefaultExecutionContext.current_parameters`
+
+            :ref:`context_default_functions`
+
+        """
+        try:
+            parameters = self.current_parameters
+            column = self.current_column
+        except AttributeError:
+            raise exc.InvalidRequestError(
+                "get_current_parameters() can only be invoked in the "
+                "context of a Python side column default function")
+        if isolate_multiinsert_groups and \
+            self.isinsert and \
+                self.compiled.statement._has_multi_parameters:
+            if column._is_multiparam_column:
+                index = column.index + 1
+                d = {column.original.key: parameters[column.key]}
+            else:
+                d = {column.key: parameters[column.key]}
+                index = 0
+            keys = self.compiled.statement.parameters[0].keys()
+            d.update(
+                (key, parameters["%s_m%d" % (key, index)])
+                for key in keys
+            )
+            return d
+        else:
+            return parameters
+
     def get_insert_default(self, column):
         if column.default is None:
             return None
         else:
-            return self._exec_default(column.default, column.type)
+            return self._exec_default(column, column.default, column.type)
 
     def get_update_default(self, column):
         if column.onupdate is None:
             return None
         else:
-            return self._exec_default(column.onupdate, column.type)
+            return self._exec_default(column, column.onupdate, column.type)
 
     def _process_executemany_defaults(self):
         key_getter = self.compiled._key_getters_for_crud_column[2]
diff --git a/lib/sqlalchemy/sql/crud.py b/lib/sqlalchemy/sql/crud.py
index 5739c22f9..8421b1e66 100644
--- a/lib/sqlalchemy/sql/crud.py
+++ b/lib/sqlalchemy/sql/crud.py
@@ -395,7 +395,10 @@ def _create_update_prefetch_bind_param(compiler, c, process=True, name=None):
 
 
 class _multiparam_column(elements.ColumnElement):
+    _is_multiparam_column = True
+
     def __init__(self, original, index):
+        self.index = index
         self.key = "%s_m%d" % (original.key, index + 1)
         self.original = original
         self.default = original.default
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 9213d616c..3b2bcb4ff 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -3656,6 +3656,8 @@ class ColumnClause(Immutable, ColumnElement):
 
     onupdate = default = server_default = server_onupdate = None
 
+    _is_multiparam_column = False
+
     _memoized_property = util.group_expirable_memoized_property()
 
     def __init__(self, text, type_=None, is_literal=False, _selectable=None):