sql server: fix writing to tmp tables with FreeTDS (#886)

* sql server: fix writing to tmp tables with FreeTDS

* add NEWS entry

* sql-server: add method to plug s4 dispatch

NEWS.md

@@ -1,5 +1,8 @@
 # odbc (development version)
 
+* SQL Server: Fix data truncation when writing to temp tables with
+  `FreeTDS` (#866).
+
 * SQL Server: Writing to DATETIME2 targets respects precision (#793).
 
 * Addressed issue where error messages rethrown from some drivers would be

R/driver-sql-server.R

@@ -214,3 +214,50 @@ setMethod("odbcDataType", "Microsoft SQL Server",
     )
   }
 )
+
+#' @description
+#' ## `odbcConnectionColumns_()`
+#'
+#' If temp table, query the database for the
+#' actual table name.
+#' @rdname SQLServer
+#' @usage NULL
+setMethod("odbcConnectionColumns_", c("Microsoft SQL Server", "character"),
+  function(conn,
+           name,
+           ...,
+           catalog_name = NULL,
+           schema_name = NULL,
+           column_name = NULL,
+           exact = FALSE) {
+    if (isTempTable(conn, name, catalog_name, schema_name, column_name, exact)) {
+      catalog_name <- "tempdb"
+      schema_name <- "dbo"
+      query <- paste0("SELECT name FROM tempdb.sys.tables WHERE ",
+        "object_id = OBJECT_ID('tempdb..", name, "')")
+      name <- dbGetQuery(conn, query)[[1]]
+    }
+
+    callNextMethod(
+      conn = conn,
+      name = name,
+      ...,
+      catalog_name = catalog_name,
+      schema_name = schema_name,
+      column_name = column_name,
+      exact = exact
+    )
+  }
+)
+
+#' @description
+#' ## `odbcConnectionColumns_()`
+#'
+#' Copied over from odbc-connection to avoid S4 dispatch NOTEs.
+#' @rdname SQLServer
+#' @usage NULL
+setMethod("odbcConnectionColumns_", c("Microsoft SQL Server", "SQL"),
+  function(conn, name, ...) {
+    odbcConnectionColumns_(conn, dbUnquoteIdentifier(conn, name)[[1]], ...)
+  }
+)