Add JSON support for mssql

Added support for the :class:`_types.JSON` datatype on the SQL Server
dialect using the :class:`_mssql.JSON` implementation, which implements SQL
Server's JSON functionality against the ``NVARCHAR(max)`` datatype as per
SQL Server documentation. Implementation courtesy Gord Thompson.

Fixes: #4384
Change-Id: I28af79a4d8fafaa68ea032228609bba727784f18

1 files changed, 125 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/mssql/json.py b/lib/sqlalchemy/dialects/mssql/json.py
new file mode 100644
index 000000000..2cae3f774
--- /dev/null
+++ b/lib/sqlalchemy/dialects/mssql/json.py
@@ -0,0 +1,125 @@
+from ... import types as sqltypes
+
+# technically, all the dialect-specific datatypes that don't have any special
+# behaviors would be private with names like _MSJson. However, we haven't been
+# doing this for mysql.JSON or sqlite.JSON which both have JSON / JSONIndexType
+# / JSONPathType in their json.py files, so keep consistent with that
+# sub-convention for now.  A future change can update them all to be
+# package-private at once.
+
+
+class JSON(sqltypes.JSON):
+    """MSSQL JSON type.
+
+    MSSQL supports JSON-formatted data as of SQL Server 2016.
+
+    The :class:`_mssql.JSON` datatype at the DDL level will represent the
+    datatype as ``NVARCHAR(max)``, but provides for JSON-level comparison
+    functions as well as Python coercion behavior.
+
+    :class:`_mssql.JSON` is used automatically whenever the base
+    :class:`_types.JSON` datatype is used against a SQL Server backend.
+
+    .. seealso::
+
+        :class:`_types.JSON` - main documenation for the generic
+        cross-platform JSON datatype.
+
+    The :class:`_mssql.JSON` type supports persistence of JSON values
+    as well as the core index operations provided by :class:`_types.JSON`
+    datatype, by adapting the operations to render the ``JSON_VALUE``
+    or ``JSON_QUERY`` functions at the database level.
+
+    The SQL Server :class:`_mssql.JSON` type necessarily makes use of the
+    ``JSON_QUERY`` and ``JSON_VALUE`` functions when querying for elements
+    of a JSON object.   These two functions have a major restriction in that
+    they are **mutually exclusive** based on the type of object to be returned.
+    The ``JSON_QUERY`` function **only** returns a JSON dictionary or list,
+    but not an individual string, numeric, or boolean element; the
+    ``JSON_VALUE`` function **only** returns an individual string, numeric,
+    or boolean element.   **both functions either return NULL or raise
+    an error if they are not used against the correct expected value**.
+
+    To handle this awkward requirement, indexed access rules are as follows:
+
+    1. When extracting a sub element from a JSON that is itself a JSON
+       dictionary or list, the :meth:`_types.JSON.Comparator.as_json` accessor
+       should be used::
+
+            stmt = select(
+                data_table.c.data["some key"].as_json()
+            ).where(
+                data_table.c.data["some key"].as_json() == {"sub": "structure"}
+            )
+
+    2. When extracting a sub element from a JSON that is a plain boolean,
+       string, integer, or float, use the appropriate method among
+       :meth:`_types.JSON.Comparator.as_boolean`,
+       :meth:`_types.JSON.Comparator.as_string`,
+       :meth:`_types.JSON.Comparator.as_integer`,
+       :meth:`_types.JSON.Comparator.as_float`::
+
+            stmt = select(
+                data_table.c.data["some key"].as_string()
+            ).where(
+                data_table.c.data["some key"].as_string() == "some string"
+            )
+
+    .. versionadded:: 1.4
+
+
+    """
+
+    # note there was a result processor here that was looking for "number",
+    # but none of the tests seem to exercise it.
+
+
+# Note: these objects currently match exactly those of MySQL, however since
+# these are not generalizable to all JSON implementations, remain separately
+# implemented for each dialect.
+class _FormatTypeMixin(object):
+    def _format_value(self, value):
+        raise NotImplementedError()
+
+    def bind_processor(self, dialect):
+        super_proc = self.string_bind_processor(dialect)
+
+        def process(value):
+            value = self._format_value(value)
+            if super_proc:
+                value = super_proc(value)
+            return value
+
+        return process
+
+    def literal_processor(self, dialect):
+        super_proc = self.string_literal_processor(dialect)
+
+        def process(value):
+            value = self._format_value(value)
+            if super_proc:
+                value = super_proc(value)
+            return value
+
+        return process
+
+
+class JSONIndexType(_FormatTypeMixin, sqltypes.JSON.JSONIndexType):
+    def _format_value(self, value):
+        if isinstance(value, int):
+            value = "$[%s]" % value
+        else:
+            value = '$."%s"' % value
+        return value
+
+
+class JSONPathType(_FormatTypeMixin, sqltypes.JSON.JSONPathType):
+    def _format_value(self, value):
+        return "$%s" % (
+            "".join(
+                [
+                    "[%s]" % elem if isinstance(elem, int) else '."%s"' % elem
+                    for elem in value
+                ]
+            )
+        )