diff --git a/pom.xml b/pom.xml
index 411e873..b40d92a 100644
--- a/pom.xml
+++ b/pom.xml
@@ -4,7 +4,7 @@
     <modelVersion>4.0.0</modelVersion>
     <groupId>io.frictionlessdata</groupId>
     <artifactId>datapackage-java</artifactId>
-    <version>0.6.11-SNAPSHOT</version>
+    <version>0.6.12-SNAPSHOT</version>
     <packaging>jar</packaging>
     <issueManagement>
         <url>https://github.com/frictionlessdata/datapackage-java/issues</url>
@@ -22,7 +22,7 @@
         <java.version>8</java.version>
         <maven.compiler.source>${java.version}</maven.compiler.source>
         <maven.compiler.target>${java.version}</maven.compiler.target>
-        <tableschema-java-version>0.6.11</tableschema-java-version>
+        <tableschema-java-version>0.6.12</tableschema-java-version>
         <hamcrest.version>1.3</hamcrest.version>
         <junit.version>5.9.1</junit.version>
         <slf4j-simple.version>2.0.5</slf4j-simple.version>
@@ -230,19 +230,21 @@
         </dependency>
 
         <!-- tableschema-java -->
+        <!--
         <dependency>
             <groupId>com.github.frictionlessdata</groupId>
             <artifactId>tableschema-java</artifactId>
             <version>${tableschema-java-version}</version>
         </dependency>
-<!--
+        -->
+
                 <dependency>
                     <groupId>io.frictionlessdata</groupId>
                     <artifactId>tableschema-java</artifactId>
-                    <version>0.6.2-SNAPSHOT</version>
+                    <version>0.6.12-SNAPSHOT</version>
                     <scope>compile</scope>
                 </dependency>
--->
+
 
         <!-- Unit Testing -->
         <dependency>
diff --git a/src/main/java/io/frictionlessdata/datapackage/Package.java b/src/main/java/io/frictionlessdata/datapackage/Package.java
index ee33d8d..3c5f970 100644
--- a/src/main/java/io/frictionlessdata/datapackage/Package.java
+++ b/src/main/java/io/frictionlessdata/datapackage/Package.java
@@ -535,7 +535,10 @@ public void write (File outputDir, boolean zipCompressed) throws Exception {
 
         for (Resource r : resourceList) {
             r.writeData(outFs.getPath(parentDirName ));
-            r.writeSchema(outFs.getPath(parentDirName));
+
+            if (null != r.getSchema()) {
+                r.writeSchema(outFs.getPath(parentDirName));
+            }
 
             // write out dialect file only if not null or URL
             String dialectP = r.getPathForWritingDialect();
diff --git a/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java b/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
index 7f7a839..1398f65 100644
--- a/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
+++ b/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
@@ -11,6 +11,7 @@
 import io.frictionlessdata.datapackage.exceptions.DataPackageException;
 import io.frictionlessdata.datapackage.exceptions.DataPackageValidationException;
 import io.frictionlessdata.tableschema.Table;
+import io.frictionlessdata.tableschema.exception.ForeignKeyException;
 import io.frictionlessdata.tableschema.fk.ForeignKey;
 import io.frictionlessdata.tableschema.io.FileReference;
 import io.frictionlessdata.tableschema.io.URLFileReference;
@@ -243,16 +244,62 @@ public List<Table> getTables() throws Exception {
         return tables;
     }
 
-    public void checkRelations() {
+    public void checkRelations(List<Resource<?,?>> resources) throws Exception {
         if (null != schema) {
             for (ForeignKey fk : schema.getForeignKeys()) {
                 fk.validate();
                 fk.getReference().validate();
             }
-            for (ForeignKey fk : schema.getForeignKeys()) {
-                if (null != fk.getReference().getResource()) {
-                    //Package pkg = new Package(fk.getReference().getDatapackage(), true);
-                    // TODO fix this
+            Iterator<Map<String, Object>> iterator = this.mappingIterator(false);
+            while (iterator.hasNext()) {
+                Map<String, Object> row = iterator.next();
+                for (ForeignKey fk : schema.getForeignKeys()) {
+                    if (null != fk.getReference().getResource()) {
+                        String targetResourceName = fk.getReference().getResource();
+                        Resource targetResoure = resources
+                                .stream()
+                                .filter((r) -> r.getName().equals(targetResourceName))
+                                .findFirst()
+                                .orElse(null);
+                        if (null == targetResoure) {
+                            throw new ForeignKeyException("Target resource " + targetResourceName + " for foreign key not found");
+                        }
+                        Iterator<Map<String, Object>> targetIterator = targetResoure.mappingIterator(false);
+                        int cnt = 0;
+                        boolean found = false;
+                        while (targetIterator.hasNext()) {
+                            Map<String, Object> targetRow = targetIterator.next();
+                            Object targetFields = fk.getReference().getFields();
+                            if (targetFields instanceof String) {
+                                if (null == targetRow.get((String) targetFields)) {
+                                    throw new ForeignKeyException("Foreign key '" + targetFields + "' violation in row \"" + cnt + "\"");
+                                }
+                                Object targetVal = targetRow.get((String)targetFields);
+                                Object val = row.get((String)fk.getFields());
+                                found = targetVal.equals(val);
+                                if (found)
+                                    break;
+                            } else if (targetFields instanceof List) {
+                                List<String> stringFields = (List<String>) targetFields;
+                                found = true;
+                                for (int idx = 0; idx < stringFields.size(); idx++) {
+                                    String targetKey = stringFields.get(idx);
+                                    if (null == targetRow.get(targetKey)) {
+                                        throw new ForeignKeyException("Foreign key '" + targetKey + "' violation in row \"" + cnt + "\"");
+                                    }
+                                    String fkKey = ((List<String>)fk.getFields()).get(idx);
+                                    found = found & (targetRow.get(targetKey).toString().equals(row.get(fkKey).toString()));
+                                }
+                                if (found){
+                                    break;
+                                }
+                            }
+                            cnt++;
+                        }
+                        if (!found) {
+                            throw new ForeignKeyException("Foreign key '" + fk.getFields() + "' violation.");
+                        }
+                    }
                 }
             }
         }
@@ -264,7 +311,6 @@ public void validate()  {
         try {
             // will validate schema against data
             tables.forEach(Table::validate);
-            checkRelations();
         } catch (Exception ex) {
             if (ex instanceof DataPackageValidationException) {
                 errors.add((DataPackageValidationException) ex);
diff --git a/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java b/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
index 01ecd6d..20b7b5c 100644
--- a/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
+++ b/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
@@ -38,16 +38,25 @@
  */
 public interface Resource<T,C> {
 
-    String FORMAT_CSV = "csv";
-    String FORMAT_JSON = "json";
+    String FORMAT_CSV = TableDataSource.Format.FORMAT_CSV.getLabel();
+    String FORMAT_JSON = TableDataSource.Format.FORMAT_JSON.getLabel();;
 
     /**
      * Return the {@link Table} objects underlying the Resource.
+     *
      * @return Table(s)
      * @throws Exception if reading the tables fails.
      */
     List<Table> getTables() throws Exception ;
 
+    /**
+     * Return a JSON representation of the Resource descriptor for use in a datapackage.json,
+     * i.e. not the JSON data
+     *
+     * See https://specs.frictionlessdata.io/data-resource/#descriptor
+     *
+     * @return JSON of the Resource description
+     */
     String getJson();
 
     /**
@@ -386,7 +395,7 @@ public interface Resource<T,C> {
 
     String getSerializationFormat();
 
-    void checkRelations() throws Exception;
+    void checkRelations(List<Resource<?,?>> resources) throws Exception;
 
     /**
      * Recreate a Resource object from a JSON descriptor, a base path to resolve relative file paths against
diff --git a/src/test/java/io/frictionlessdata/datapackage/ForeignKeysTest.java b/src/test/java/io/frictionlessdata/datapackage/ForeignKeysTest.java
index f4f9506..d9ba150 100644
--- a/src/test/java/io/frictionlessdata/datapackage/ForeignKeysTest.java
+++ b/src/test/java/io/frictionlessdata/datapackage/ForeignKeysTest.java
@@ -1,6 +1,8 @@
 package io.frictionlessdata.datapackage;
 
 import io.frictionlessdata.datapackage.resource.Resource;
+import io.frictionlessdata.tableschema.exception.ForeignKeyException;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
@@ -9,13 +11,56 @@
 public class ForeignKeysTest {
 
     @Test
-    @DisplayName("Test that a schema can be defined via a URL")
-        // Test for https://github.com/frictionlessdata/specs/issues/645
-    void testValidationURLAsSchemaReference() throws Exception{
+    @DisplayName("Test that a schema can define foreign keys and our code resolves them")
+        // Test for https://github.com/frictionlessdata/datapackage-java/issues/4
+    void testForeignKeyValidation() throws Exception{
         Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/foreign-keys.json");
         Package pkg = new Package(resourcePath, true);
-        System.out.println(pkg);
         Resource teams = pkg.getResource("teams");
-        teams.checkRelations();
+        teams.checkRelations(pkg.getResources());
+    }
+
+    @Test
+    @DisplayName("Test that a schema can define foreign keys and our code resolves them -> must throw on invalid ref")
+        // Test for https://github.com/frictionlessdata/datapackage-java/issues/4
+    void testForeignKeyValidationError() throws Exception{
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/foreign-keys-invalid.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+        ForeignKeyException fke = Assertions.assertThrows(
+                ForeignKeyException.class, () -> teams.checkRelations(pkg.getResources()));
+        Assertions.assertEquals("Foreign key 'city' violation.", fke.getMessage());
+    }
+
+    @Test
+    @DisplayName("Test that a schema can define compound foreign keys and our code resolves them")
+        // Test for https://github.com/frictionlessdata/datapackage-java/issues/4
+    void testCompoundForeignKeyValidation() throws Exception{
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/foreign-keys-extended.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+        teams.checkRelations(pkg.getResources());
+    }
+
+    @Test
+    @DisplayName("Test that a schema with mismatched String and Array foreign key must throw")
+    void testCompoundForeignKeyValidationError() throws Exception{
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/foreign-keys-extended-invalid1.json");
+        ForeignKeyException fke = Assertions.assertThrows(
+                ForeignKeyException.class, () -> new Package(resourcePath, true));
+        Assertions.assertEquals(
+                "The reference's fields property must be an array if the outer fields is an array.",
+                fke.getMessage());
+    }
+
+    @Test
+    @DisplayName("Test that a schema with mismatched String and Array foreign key must throw")
+    void testCompoundForeignKeyValidationError2() throws Exception{
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/foreign-keys-extended-invalid2.json");
+        ForeignKeyException fke = Assertions.assertThrows(
+                ForeignKeyException.class, () -> new Package(resourcePath, true));
+        Assertions.assertEquals(
+                "The reference's fields property must be a string if the outer fields is a string.",
+                fke.getMessage());
     }
 }
diff --git a/src/test/java/io/frictionlessdata/datapackage/TestUtil.java b/src/test/java/io/frictionlessdata/datapackage/TestUtil.java
index 24a1cf5..d89dbd8 100644
--- a/src/test/java/io/frictionlessdata/datapackage/TestUtil.java
+++ b/src/test/java/io/frictionlessdata/datapackage/TestUtil.java
@@ -1,9 +1,6 @@
 package io.frictionlessdata.datapackage;
 
-import java.io.ByteArrayOutputStream;
-import java.io.File;
-import java.io.IOException;
-import java.io.InputStream;
+import java.io.*;
 import java.net.URL;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -35,6 +32,9 @@ public static Path getResourcePath (String fileName) {
             }
             // Create file-URL of source file:
             URL sourceFileUrl = TestUtil.class.getResource(locFileName);
+            if (null == sourceFileUrl) {
+                throw new FileNotFoundException("Resource "+fileName+" not found");
+            }
             // Get path of URL
             return Paths.get(sourceFileUrl.toURI());
         } catch (Exception ex) {
diff --git a/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid1.json b/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid1.json
new file mode 100644
index 0000000..1eee803
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid1.json
@@ -0,0 +1,76 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "data": [
+        [
+          "id",
+          "name",
+          "city"
+        ],
+        [
+          "1",
+          "Arsenal",
+          "London"
+        ],
+        [
+          "2",
+          "Real",
+          "Madrid"
+        ],
+        [
+          "3",
+          "Bayern",
+          "Munich"
+        ]
+      ],
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer"
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "fields": ["id", "city"],
+            "reference": {
+              "resource": "cities",
+              "fields": "name"
+            }
+          }
+        ]
+      }
+    },
+    {
+      "name": "cities",
+      "data": [
+        [
+          "name",
+          "country"
+        ],
+        [
+          "London",
+          "England"
+        ],
+        [
+          "Madrid",
+          "Spain"
+        ],
+        [
+          "Munich",
+          "Germany"
+        ]
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid2.json b/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid2.json
new file mode 100644
index 0000000..8a56511
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/foreign-keys-extended-invalid2.json
@@ -0,0 +1,76 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "data": [
+        [
+          "id",
+          "name",
+          "city"
+        ],
+        [
+          "1",
+          "Arsenal",
+          "London"
+        ],
+        [
+          "2",
+          "Real",
+          "Madrid"
+        ],
+        [
+          "3",
+          "Bayern",
+          "Munich"
+        ]
+      ],
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer"
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "fields": "city",
+            "reference": {
+              "resource": "cities",
+              "fields": ["name", "country"]
+            }
+          }
+        ]
+      }
+    },
+    {
+      "name": "cities",
+      "data": [
+        [
+          "name",
+          "country"
+        ],
+        [
+          "London",
+          "England"
+        ],
+        [
+          "Madrid",
+          "Spain"
+        ],
+        [
+          "Munich",
+          "Germany"
+        ]
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/foreign-keys-extended.json b/src/test/resources/fixtures/datapackages/foreign-keys-extended.json
new file mode 100644
index 0000000..b3656ac
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/foreign-keys-extended.json
@@ -0,0 +1,80 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "data": [
+        [
+          "id",
+          "name",
+          "city"
+        ],
+        [
+          "1",
+          "Arsenal",
+          "London"
+        ],
+        [
+          "2",
+          "Real",
+          "Madrid"
+        ],
+        [
+          "3",
+          "Bayern",
+          "Munich"
+        ]
+      ],
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer"
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "fields": ["id", "city"],
+            "reference": {
+              "resource": "cities",
+              "fields": ["id","name"]
+            }
+          }
+        ]
+      }
+    },
+    {
+      "name": "cities",
+      "data": [
+        [
+          "id",
+          "name",
+          "country"
+        ],
+        [
+          "1",
+          "London",
+          "England"
+        ],
+        [
+          "2",
+          "Madrid",
+          "Spain"
+        ],
+        [
+          "3",
+          "Munich",
+          "Germany"
+        ]
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/foreign-keys-invalid.json b/src/test/resources/fixtures/datapackages/foreign-keys-invalid.json
new file mode 100644
index 0000000..e6e63db
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/foreign-keys-invalid.json
@@ -0,0 +1,72 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "data": [
+        [
+          "id",
+          "name",
+          "city"
+        ],
+        [
+          "1",
+          "Arsenal",
+          "London"
+        ],
+        [
+          "2",
+          "Real",
+          "Madrid"
+        ],
+        [
+          "3",
+          "Bayern",
+          "Munich"
+        ]
+      ],
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer"
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "foreignKeys": [
+          {
+            "fields": "city",
+            "reference": {
+              "resource": "cities",
+              "fields": "name"
+            }
+          }
+        ]
+      }
+    },
+    {
+      "name": "cities",
+      "data": [
+        [
+          "name",
+          "country"
+        ],
+        [
+          "London",
+          "England"
+        ],
+        [
+          "Madrid",
+          "Spain"
+        ]
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/foreign-keys.json b/src/test/resources/fixtures/datapackages/foreign-keys.json
index e6e63db..cb5559f 100644
--- a/src/test/resources/fixtures/datapackages/foreign-keys.json
+++ b/src/test/resources/fixtures/datapackages/foreign-keys.json
@@ -65,6 +65,10 @@
         [
           "Madrid",
           "Spain"
+        ],
+        [
+          "Munich",
+          "Germany"
         ]
       ]
     }

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