[WIP] Experimental external foreign keys support

data/datapackages_linked/cities/cities.csv

@@ -0,0 +1,4 @@
+id,city
+1,london
+2,paris
+3,rome

data/datapackages_linked/cities/datapackage.json

@@ -0,0 +1,30 @@
+{
+  "profile": "tabular-data-package",
+  "resources": [
+    {
+      "path": "cities.csv",
+      "profile": "tabular-data-resource",
+      "name": "cities",
+      "format": "csv",
+      "mediatype": "text/csv",
+      "encoding": "utf-8",
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer",
+            "format": "default"
+          },
+          {
+            "name": "city",
+            "type": "string",
+            "format": "default"
+          }
+        ],
+        "missingValues": [
+          ""
+        ]
+      }
+    }
+  ]
+}

data/datapackages_linked/people/datapackage.json

@@ -0,0 +1,40 @@
+{
+  "profile": "tabular-data-package",
+  "resources": [
+    {
+      "path": "people.csv",
+      "profile": "tabular-data-resource",
+      "name": "people",
+      "format": "csv",
+      "mediatype": "text/csv",
+      "encoding": "utf-8",
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer",
+            "format": "default"
+          },
+          {
+            "name": "population",
+            "type": "integer",
+            "format": "default"
+          }
+        ],
+        "missingValues": [
+          ""
+        ],
+        "foreignKeys": [
+            {
+                "fields": "id",
+                "reference": {
+                    "package": "../cities/datapackage.json",
+                    "resource": "cities",
+                    "fields": "id"
+                }
+            }
+        ]
+      }
+    }
+  ]
+}

data/datapackages_linked/people/people.csv

@@ -0,0 +1,4 @@
+id,population
+1,8
+2,2
+4,3

datapackage/resource.py

@@ -398,24 +398,44 @@ def __get_table(self):
 
     def __get_relations(self):
         if not self.__relations:
+            table = self.__get_table()
 
             # Prepare resources
             resources = {}
-            if self.__get_table() and self.__get_table().schema:
-                for fk in self.__get_table().schema.foreign_keys:
-                    resources.setdefault(fk['reference']['resource'], [])
-                    for field in fk['reference']['fields']:
-                        resources[fk['reference']['resource']].append(field)
+            if table and table.schema:
+                for fk in table.schema.foreign_keys:
+                    resource_name = fk['reference'].get('resource')
+                    package_name = fk['reference'].get('package')
+
+                    # Self-referenced resource
+                    if not resource_name:
+                        resource = self
+
+                    # Internal resource
+                    elif not package_name:
+                        if not self.__package:
+                            continue
+                        resource = self.__package.get_resource(resource_name)
+
+                    # External resource (experimental)
+                    # For now, we rely on uniqueness of resource names and relative paths
+                    else:
+                        from .package import Package
+                        package = Package('/'.join([self.__base_path, package_name]))
+                        resource = package.get_resource(resource_name)
+
+                    # Check/add resource
+                    if not resource:
+                        message = 'Foreign key "%s" violation: resource not found'
+                        message = message % fk['fields']
+                        raise exceptions.RelationError(message)
+                    resources[resource_name] = resource
 
             # Fill relations
             self.__relations = {}
-            for resource, fields in resources.items():
-                if resource and not self.__package:
-                    continue
-                self.__relations.setdefault(resource, [])
-                data = self.__package.get_resource(resource) if resource else self
-                if data.tabular:
-                    self.__relations[resource] = data.read(keyed=True)
+            for resource_name, resource in resources.items():
+                if resource.tabular:
+                    self.__relations[resource_name] = resource.read(keyed=True)
 
         return self.__relations