Improve string support for reporter expressions

tests/reporter/group_date_composition_test.go

@@ -0,0 +1,27 @@
+package reporter
+
+import (
+	"testing"
+
+	"github.com/cortezaproject/corteza-server/pkg/report"
+)
+
+func Test_group_date_composition(t *testing.T) {
+	var (
+		ctx, h, s = setup(t)
+		m, _, dd  = loadScenario(ctx, s, t, h)
+		ff        []*report.Frame
+		def       = dd[0]
+	)
+
+	ff = loadNoErr(ctx, h, m, def)
+	h.a.Len(ff, 1)
+	r := ff[0]
+	h.a.Equal(2, r.Size())
+
+	h.a.Equal("by_dob<String>, count<Number>", r.Columns.String())
+
+	checkRows(h, ff[0],
+		"2020-01, 5",
+		"2021-01, 7")
+}

tests/reporter/testdata/group_date_composition/report.json

@@ -0,0 +1,38 @@
+{
+  "handle": "testing_report",
+  "sources": [
+    { "step": { "load": {
+      "name": "users",
+      "source": "composeRecords",
+      "definition": {
+        "module": "user",
+        "namespace": "ns"
+      }
+    }}},
+
+    { "step": { "group": {
+      "name": "grouped",
+      "source": "users",
+      "keys": [
+        { "name": "by_dob", "def": "concat(string(year(dob)), '-', string(month(dob)))" }
+      ],
+      "columns": [
+        {
+          "name": "count",
+          "def": {
+            "ref": "count"
+          }
+        }
+      ]
+    }}}
+  ],
+  "frames": [{
+    "name":   "result",
+    "source": "grouped",
+    "columns": [
+      { "name": "by_dob", "label": "by_dob" },
+      { "name": "count", "label": "count" }
+    ],
+    "sort": "by_dob ASC"
+  }]
+}