Ensure foreign key columns create an index by default and add tests for constrained method behavior

src/Illuminate/Database/Schema/ForeignIdColumnDefinition.php

@@ -39,6 +39,12 @@ public function constrained($table = null, $column = null, $indexName = null)
         $table ??= $this->table;
         $column ??= $this->referencesModelColumn ?? 'id';
 
+        // Ensure an index is created for the foreign key column
+        // This is important for query performance, especially on PostgreSQL
+        if (! isset($this->index)) {
+            $this->index = true;
+        }
+
         return $this->references($column, $indexName)->on($table ?? (new Stringable($this->name))->beforeLast('_'.$column)->plural());
     }
 

tests/Database/DatabaseForeignIdColumnDefinitionTest.php

@@ -0,0 +1,106 @@
+<?php
+
+namespace Illuminate\Tests\Database;
+
+use Illuminate\Database\Connection;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Schema\Grammars\PostgresGrammar;
+use Mockery as m;
+use PHPUnit\Framework\TestCase;
+
+class DatabaseForeignIdColumnDefinitionTest extends TestCase
+{
+    protected function tearDown(): void
+    {
+        m::close();
+    }
+
+    public function testConstrainedMethodSetsIndexProperty()
+    {
+        $connection = m::mock(Connection::class);
+        $connection->shouldReceive('getSchemaGrammar')->andReturn(new PostgresGrammar($connection));
+        $connection->shouldReceive('getTablePrefix')->andReturn('');
+        $connection->shouldReceive('getConfig')->with('prefix_indexes')->andReturn(false);
+
+        $blueprint = new Blueprint($connection, 'posts');
+        $column = $blueprint->foreignId('user_id');
+        $this->assertObjectNotHasProperty('index', $column);
+        $column->constrained();
+        $this->assertTrue($column->index);
+    }
+
+    public function testConstrainedPreservesExplicitIndexFalse()
+    {
+        $connection = m::mock(Connection::class);
+        $connection->shouldReceive('getSchemaGrammar')->andReturn(new PostgresGrammar($connection));
+        $connection->shouldReceive('getTablePrefix')->andReturn('');
+        $connection->shouldReceive('getConfig')->with('prefix_indexes')->andReturn(false);
+
+        $blueprint = new Blueprint($connection, 'posts');
+        $column = $blueprint->foreignId('user_id');
+        $column->index = false;
+        $column->constrained();
+        $this->assertFalse($column->index);
+    }
+
+    public function testAddFluentIndexesCreatesIndexForForeignId()
+    {
+        $connection = m::mock(Connection::class);
+        $grammar = m::mock(PostgresGrammar::class);
+        $connection->shouldReceive('getSchemaGrammar')->andReturn($grammar);
+        $connection->shouldReceive('getTablePrefix')->andReturn('');
+        $connection->shouldReceive('getConfig')->with('prefix_indexes')->andReturn(false);
+        $grammar->shouldReceive('getFluentCommands')->andReturn([]);
+        $grammar->shouldReceive('compileAdd')->andReturn('');
+        $grammar->shouldReceive('compileForeign')->andReturn('');
+        $grammar->shouldReceive('compileIndex')->andReturn('');
+
+        $blueprint = new Blueprint($connection, 'posts');
+        $blueprint->foreignId('user_id')->constrained();
+        $blueprint->toSql();
+        $commands = $blueprint->getCommands();
+        $indexCommands = array_filter($commands, function ($command) {
+            return $command->name === 'index' && in_array('user_id', (array) $command->columns);
+        });
+
+        $this->assertNotEmpty($indexCommands, 'An index command should be created for the foreign key column');
+    }
+
+    public function testConstrainedWithCompositeUniqueStillCreatesIndex()
+    {
+        $connection = m::mock(Connection::class);
+        $grammar = m::mock(PostgresGrammar::class);
+        $connection->shouldReceive('getSchemaGrammar')->andReturn($grammar);
+        $connection->shouldReceive('getTablePrefix')->andReturn('');
+        $connection->shouldReceive('getConfig')->with('prefix_indexes')->andReturn(false);
+        $grammar->shouldReceive('getFluentCommands')->andReturn([]);
+        $grammar->shouldReceive('compileAdd')->andReturn('');
+        $grammar->shouldReceive('compileForeign')->andReturn('');
+        $grammar->shouldReceive('compileUnique')->andReturn('');
+        $grammar->shouldReceive('compileIndex')->andReturn('');
+
+        $blueprint = new Blueprint($connection, 'books');
+        $blueprint->id();
+        $blueprint->foreignId('author_id')->constrained();
+        $blueprint->foreignId('category_id')->constrained();
+        $blueprint->string('title');
+        $blueprint->unique(['author_id', 'title']);
+        $blueprint->toSql();
+        $commands = $blueprint->getCommands();
+        $authorIndexCommands = array_filter($commands, function ($command) {
+            return $command->name === 'index' &&
+                is_array($command->columns) &&
+                count($command->columns) === 1 &&
+                $command->columns[0] === 'author_id';
+        });
+        $categoryIndexCommands = array_filter($commands, function ($command) {
+            return $command->name === 'index' &&
+                is_array($command->columns) &&
+                count($command->columns) === 1 &&
+                $command->columns[0] === 'category_id';
+        });
+
+        $this->assertNotEmpty($authorIndexCommands, 'Index should be created for author_id despite composite unique');
+        $this->assertNotEmpty($categoryIndexCommands, 'Index should be created for category_id');
+    }
+}