Allow a way to insert demo data only for empty table.

docs/en/seeding.rst

@@ -138,7 +138,7 @@ within your seed class and then use the `insert()` method to insert data:
                     ],[
                         'body'    => 'bar',
                         'created' => date('Y-m-d H:i:s'),
-                    ]
+                    ],
                 ];
 
                 $posts = $this->table('posts');
@@ -193,6 +193,21 @@ Then use it in your seed classes:
             }
         }
 
+Executing seeds only once
+-------------------------
+
+If you want to make sure your seed data doesn't get added multiple times, a
+basic check on if the table is empty or contains already some data can help.
+
+.. code-block:: php
+    public function run()
+    {
+        if ($this->hasData('posts')) {
+            return;
+        }
+        ...
+    }
+
 Truncating Tables
 -----------------
 

src/Phinx/Seed/AbstractSeed.php

@@ -162,11 +162,21 @@ public function fetchAll($sql)
      */
     public function insert($table, $data)
     {
-        // convert to table object
-        if (is_string($table)) {
-            $table = new Table($table, [], $this->getAdapter());
-        }
-        $table->insert($data)->save();
+        $tableInstance = new Table($table, [], $this->getAdapter());
+        $tableInstance->insert($data)->save();
+    }
+
+    /**
+     * @param string $tableName
+     *
+     * @return bool
+     */
+    public function hasData(string $tableName): bool
+    {
+        $countQuery = $this->getAdapter()->query('SELECT COUNT(*) FROM ' . $tableName);
+        $res = $countQuery->fetchAll();
+
+        return $res[0]['count'] > 0;
     }
 
     /**

tests/Phinx/Seed/SeedTest.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Test\Phinx\Seed;
+
+use PDOStatement;
+use Phinx\Db\Adapter\PdoAdapter;
+use Phinx\Seed\AbstractSeed;
+use PHPUnit\Framework\TestCase;
+
+class SeedTest extends TestCase
+{
+    /**
+     * @return void
+     */
+    public function testHasData(): void
+    {
+        $queryStub = $this->getMockBuilder(PDOStatement::class)->disableOriginalConstructor()->getMock();
+        $queryStub->expects($this->once())
+            ->method('fetchAll')
+            ->will($this->returnValue([0 => ['count' => 0]]));
+
+        $adapterStub = $this->getMockBuilder(PdoAdapter::class)
+            ->setConstructorArgs([[]])
+            ->getMock();
+        $adapterStub->expects($this->once())
+            ->method('query')
+            ->will($this->returnValue($queryStub));
+
+        $stub = $this->getMockForAbstractClass(AbstractSeed::class);
+        $stub->setAdapter($adapterStub);
+        $result = $stub->hasData('foo');
+
+        $this->assertFalse($result);
+    }
+
+    /**
+     * @return void
+     */
+    public function testHasDataTrue(): void
+    {
+        $queryStub = $this->getMockBuilder(PDOStatement::class)->disableOriginalConstructor()->getMock();
+        $queryStub->expects($this->once())
+            ->method('fetchAll')
+            ->will($this->returnValue([0 => ['count' => 1]]));
+
+        $adapterStub = $this->getMockBuilder(PdoAdapter::class)
+            ->setConstructorArgs([[]])
+            ->getMock();
+        $adapterStub->expects($this->once())
+            ->method('query')
+            ->will($this->returnValue($queryStub));
+
+        $stub = $this->getMockForAbstractClass(AbstractSeed::class);
+        $stub->setAdapter($adapterStub);
+        $result = $stub->hasData('foo');
+
+        $this->assertTrue($result);
+    }
+}