diff --git a/layouts/partials/decap-cms/functions/compute-definition.html b/layouts/partials/decap-cms/functions/compute-definition.html
new file mode 100644
index 0000000..6240d79
--- /dev/null
+++ b/layouts/partials/decap-cms/functions/compute-definition.html
@@ -0,0 +1,26 @@
+{{- $definitions := .definitions -}}
+{{- $name := .name -}}
+{{- $def := $definitions.Get $name -}}
+{{- $extends := default slice (index $def "_extends") -}}
+{{- if not (reflect.IsSlice $extends) }}
+  {{- errorf "[decap-cms] %q _extends is not an array." $name }}
+{{- end }}
+{{- $merging := slice -}}
+{{- range $extends -}}
+  {{- $parentName := . -}}
+  {{- with $definitions.Get $parentName -}}
+    {{- $parent := . -}}
+    {{/* Handle nested extends. */}}
+    {{- with index $parent "_extends" -}}
+      {{- partial "decap-cms/functions/compute-definition" (dict "definitions" $definitions "name" $parentName) -}}
+    {{- end -}}
+    {{- $merging = $merging | append ($definitions.Get $parentName) -}}
+  {{- else -}}
+    {{- errorf "[decap-cms] no such extensible config: %s" $name -}}
+  {{- end -}}
+{{- end -}}
+{{/* Override with the computed definition. */}}
+{{- with $merging -}}
+  {{- $def = partial "decap-cms/functions/deep-merge" (. | append $def) -}}
+  {{- $definitions.Set $name $def.Values -}}
+{{- end -}}
diff --git a/layouts/partials/decap-cms/functions/config.html b/layouts/partials/decap-cms/functions/config.html
index 7f03c5a..fadc8c3 100644
--- a/layouts/partials/decap-cms/functions/config.html
+++ b/layouts/partials/decap-cms/functions/config.html
@@ -11,12 +11,10 @@
     {{- $config.Set $key (index $params $key) -}}
   {{- end -}}
 {{- end -}}
-{{- $configs := default dict (index $params "_configs") -}}
-
+{{- $definitions := partial "decap-cms/functions/definitions" (default dict (index $params "_configs")) }}
 {{/* Transform collections. */}}
 {{- with index $params "collections" -}}
-  {{- range . -}}
-    {{- $collection := . -}}
+  {{- range $collectionName, $collection := . -}}
     {{- with $collection.fields -}}
       {{- if reflect.IsSlice . -}}
         {{- $fields := dict -}}
@@ -30,23 +28,20 @@
     {{/* Extends configurations. */}}
     {{- $extends := slice -}}
     {{- with index $collection "_extends" -}}
+      {{- if not (reflect.IsSlice .) }}
+        {{- errorf "[decap-cms] %q _extends is not an array." $collectionName }}
+      {{- end }}
       {{- range $weight, $extendName := . -}}
-        {{- with index $configs . -}}
+        {{- with index $definitions . -}}
           {{/* Assign the weight to fields for sorting. */}}
           {{- $extend := . -}}
           {{- with $extend.fields -}}
-            {{- $isMap := reflect.IsMap . }}
-            {{- range $name, $field := . -}}
-              {{- $field = merge $field (dict "_weight" (add $weight 1)) -}}
-              {{- if not $isMap }}
-                {{- $name = $field.name }}
-              {{- end }}
-              {{- $extend = merge $extend (dict "fields" (dict $name $field)) -}}
-            {{- end -}}
+            {{- $fields := partial "decap-cms/functions/format-fields" . -}}
+            {{- $extend = merge $extend (dict "fields" $fields) -}}
           {{- end -}}
           {{- $extends = $extends | append $extend -}}
         {{- else -}}
-          {{- warnf "[decap-cms] no such extendable configuration: %s." . -}}
+          {{- errorf "[decap-cms] no such extendable configuration: %s." . -}}
         {{- end -}}
       {{- end -}}
     {{- end -}}
diff --git a/layouts/partials/decap-cms/functions/deep-merge.html b/layouts/partials/decap-cms/functions/deep-merge.html
index f51ca26..7c16faf 100644
--- a/layouts/partials/decap-cms/functions/deep-merge.html
+++ b/layouts/partials/decap-cms/functions/deep-merge.html
@@ -37,6 +37,22 @@
         {{- $rst.Set $key $values }}
       {{- else }}
         {{/* Start map merging. */}}
+        {{- if eq $key "fields" }}
+          {{/* Calculate fields weight. */}}
+          {{- $fieldWeight := len $values }}
+          {{- $fields := newScratch }}
+          {{- range $fieldName, $field := $items }}
+            {{- $field = partial "decap-cms/functions/dict2scratch" $field }}
+            {{- if isset $values $fieldName }}
+              {{- $field.Delete "_weight" }}
+            {{- else }}
+              {{- $fieldWeight = add $fieldWeight 1 }}
+              {{- $field.Set "_weight" $fieldWeight }}
+            {{- end }}
+            {{- $fields.Set $fieldName $field.Values }}
+          {{- end }}
+          {{- $items = $fields.Values }}
+        {{- end }}
         {{- $rst.Set $key (merge $values $items) }}
       {{- end }}
     {{- else }}
diff --git a/layouts/partials/decap-cms/functions/definitions.html b/layouts/partials/decap-cms/functions/definitions.html
new file mode 100644
index 0000000..bbdf000
--- /dev/null
+++ b/layouts/partials/decap-cms/functions/definitions.html
@@ -0,0 +1,13 @@
+{{- $definitions := newScratch -}}
+{{/* Convert definitions from map to scratch. */}}
+{{- range $name, $def := . -}}
+  {{- with $def.fields -}}
+    {{- $fields := partial "decap-cms/functions/format-fields" . -}}
+    {{- $def = merge $def (dict "fields" $fields) -}}
+  {{- end -}}
+  {{- $definitions.Set $name $def -}}
+{{- end -}}
+{{- range $name, $def := . -}}
+  {{- partial "decap-cms/functions/compute-definition" (dict "definitions" $definitions "name" $name) -}}
+{{- end -}}
+{{- return $definitions.Values -}}
diff --git a/layouts/partials/decap-cms/functions/dict2scratch.html b/layouts/partials/decap-cms/functions/dict2scratch.html
index 3add2f1..f80c27b 100644
--- a/layouts/partials/decap-cms/functions/dict2scratch.html
+++ b/layouts/partials/decap-cms/functions/dict2scratch.html
@@ -1,5 +1,5 @@
 {{- $scratch := newScratch }}
-{{- range $key, $val := index . 0 }}
+{{- range $key, $val := . }}
   {{- $scratch.Set $key $val }}
 {{- end }}
 {{- return $scratch }}
diff --git a/layouts/partials/decap-cms/functions/format-fields.html b/layouts/partials/decap-cms/functions/format-fields.html
new file mode 100644
index 0000000..1d95f41
--- /dev/null
+++ b/layouts/partials/decap-cms/functions/format-fields.html
@@ -0,0 +1,17 @@
+{{/* Format fields to add _weight and convert it to map if it's an array. */}}
+{{- $fields := newScratch -}}
+{{- with . -}}
+  {{- $isMap := reflect.IsMap . -}}
+  {{- $weight := 0 -}}
+  {{- range $name, $field := . -}}
+    {{- if isset $field "_weight" }}
+      {{- continue }}
+    {{- end }}
+    {{- $field = merge $field (dict "_weight" (add $weight 1)) -}}
+    {{- if not $isMap -}}
+      {{- $name = $field.name -}}
+    {{- end -}}
+    {{- $fields.Set $name $field -}}
+  {{- end -}}
+{{- end -}}
+{{- return $fields.Values -}}