[11.x] Support eager loading with limit

src/Illuminate/Database/Eloquent/Relations/BelongsToMany.php

@@ -11,6 +11,7 @@
 use Illuminate\Database\Eloquent\Relations\Concerns\AsPivot;
 use Illuminate\Database\Eloquent\Relations\Concerns\InteractsWithDictionary;
 use Illuminate\Database\Eloquent\Relations\Concerns\InteractsWithPivotTable;
+use Illuminate\Database\Query\Grammars\MySqlGrammar;
 use Illuminate\Database\UniqueConstraintViolationException;
 use Illuminate\Support\Str;
 use InvalidArgumentException;
@@ -1347,6 +1348,42 @@ public function getRelationExistenceQueryForSelfJoin(Builder $query, Builder $pa
         return parent::getRelationExistenceQuery($query, $parentQuery, $columns);
     }
 
+    /**
+     * Alias to set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function take($value)
+    {
+        return $this->limit($value);
+    }
+
+    /**
+     * Set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function limit($value)
+    {
+        if ($this->parent->exists) {
+            $this->query->limit($value);
+        } else {
+            $column = $this->getExistenceCompareKey();
+
+            $grammar = $this->query->getQuery()->getGrammar();
+
+            if ($grammar instanceof MySqlGrammar && $grammar->useLegacyGroupLimit($this->query->getQuery())) {
+                $column = 'pivot_'.last(explode('.', $column));
+            }
+
+            $this->query->groupLimit($value, $column);
+        }
+
+        return $this;
+    }
+
     /**
      * Get the key for comparing against the parent key in "has" query.
      *

src/Illuminate/Database/Eloquent/Relations/HasManyThrough.php

@@ -10,6 +10,7 @@
 use Illuminate\Database\Eloquent\ModelNotFoundException;
 use Illuminate\Database\Eloquent\Relations\Concerns\InteractsWithDictionary;
 use Illuminate\Database\Eloquent\SoftDeletes;
+use Illuminate\Database\Query\Grammars\MySqlGrammar;
 use Illuminate\Database\UniqueConstraintViolationException;
 
 class HasManyThrough extends Relation
@@ -762,6 +763,42 @@ public function getRelationExistenceQueryForThroughSelfRelation(Builder $query,
         );
     }
 
+    /**
+     * Alias to set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function take($value)
+    {
+        return $this->limit($value);
+    }
+
+    /**
+     * Set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function limit($value)
+    {
+        if ($this->farParent->exists) {
+            $this->query->limit($value);
+        } else {
+            $column = $this->getQualifiedFirstKeyName();
+
+            $grammar = $this->query->getQuery()->getGrammar();
+
+            if ($grammar instanceof MySqlGrammar && $grammar->useLegacyGroupLimit($this->query->getQuery())) {
+                $column = 'laravel_through_key';
+            }
+
+            $this->query->groupLimit($value, $column);
+        }
+
+        return $this;
+    }
+
     /**
      * Get the qualified foreign key on the related model.
      *

src/Illuminate/Database/Eloquent/Relations/HasOneOrMany.php

@@ -453,6 +453,34 @@ public function getRelationExistenceQueryForSelfRelation(Builder $query, Builder
         );
     }
 
+    /**
+     * Alias to set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function take($value)
+    {
+        return $this->limit($value);
+    }
+
+    /**
+     * Set the "limit" value of the query.
+     *
+     * @param  int  $value
+     * @return $this
+     */
+    public function limit($value)
+    {
+        if ($this->parent->exists) {
+            $this->query->limit($value);
+        } else {
+            $this->query->groupLimit($value, $this->getExistenceCompareKey());
+        }
+
+        return $this;
+    }
+
     /**
      * Get the key for comparing against the parent key in "has" query.
      *

src/Illuminate/Database/Query/Builder.php

@@ -152,6 +152,13 @@ class Builder implements BuilderContract
      */
     public $limit;
 
+    /**
+     * The maximum number of records to return per group.
+     *
+     * @var array
+     */
+    public $groupLimit;
+
     /**
      * The number of records to skip.
      *
@@ -2444,6 +2451,22 @@ public function limit($value)
         return $this;
     }
 
+    /**
+     * Add a "group limit" clause to the query.
+     *
+     * @param  int  $value
+     * @param  string  $column
+     * @return $this
+     */
+    public function groupLimit($value, $column)
+    {
+        if ($value >= 0) {
+            $this->groupLimit = compact('value', 'column');
+        }
+
+        return $this;
+    }
+
     /**
      * Set the limit and offset for a given page.
      *
@@ -2737,9 +2760,30 @@ public function soleValue($column)
      */
     public function get($columns = ['*'])
     {
-        return collect($this->onceWithColumns(Arr::wrap($columns), function () {
+        $items = collect($this->onceWithColumns(Arr::wrap($columns), function () {
             return $this->processor->processSelect($this, $this->runSelect());
         }));
+
+        if (! isset($this->groupLimit)) {
+            return $items;
+        }
+
+        $keysToRemove = ['laravel_row'];
+
+        if (is_string($this->groupLimit['column'])) {
+            $column = last(explode('.', $this->groupLimit['column']));
+
+            $keysToRemove[] = '@laravel_group := '.$this->grammar->wrap($column);
+            $keysToRemove[] = '@laravel_group := '.$this->grammar->wrap('pivot_'.$column);
+        }
+
+        $items->each(function ($item) use ($keysToRemove) {
+            foreach ($keysToRemove as $key) {
+                unset($item->$key);
+            }
+        });
+
+        return $items;
     }
 
     /**

src/Illuminate/Database/Query/Grammars/Grammar.php

@@ -60,6 +60,14 @@ public function compileSelect(Builder $query)
             return $this->compileUnionAggregate($query);
         }
 
+        if (isset($query->groupLimit)) {
+            if (is_null($query->columns)) {
+                $query->columns = ['*'];
+            }
+
+            return $this->compileGroupLimit($query);
+        }
+
         // If the query does not have any columns set, we'll set the columns to the
         // * character to just get all of the columns from the database. Then we
         // can build the query and concatenate all the pieces together as one.
@@ -917,6 +925,68 @@ protected function compileLimit(Builder $query, $limit)
         return 'limit '.(int) $limit;
     }
 
+    /**
+     * Compile a group limit clause.
+     *
+     * @param  \Illuminate\Database\Query\Builder  $query
+     * @return string
+     */
+    protected function compileGroupLimit(Builder $query)
+    {
+        $selectBindings = array_merge($query->getRawBindings()['select'], $query->getRawBindings()['order']);
+
+        $query->setBindings($selectBindings, 'select');
+        $query->setBindings([], 'order');
+
+        $limit = (int) $query->groupLimit['value'];
+
+        $offset = $query->offset;
+
+        if (isset($offset)) {
+            $offset = (int) $offset;
+            $limit += $offset;
+
+            $query->offset = null;
+        }
+
+        $components = $this->compileComponents($query);
+
+        $components['columns'] .= $this->compileRowNumber($query->groupLimit['column'], $components['orders'] ?? '');
+
+        unset($components['orders']);
+
+        $table = $this->wrap('laravel_table');
+        $row = $this->wrap('laravel_row');
+
+        $sql = $this->concatenate($components);
+
+        $sql = 'select * from ('.$sql.') as '.$table.' where '.$row.' <= '.$limit;
+
+        if (isset($offset)) {
+            $sql .= ' and '.$row.' > '.$offset;
+        }
+
+        return $sql.' order by '.$row;
+    }
+
+    /**
+     * Compile a row number clause.
+     *
+     * @param  string  $partition
+     * @param  string  $orders
+     * @return string
+     */
+    protected function compileRowNumber($partition, $orders)
+    {
+        $partition = 'partition by '.$this->wrap($partition);
+
+        $over = trim($partition.' '.$orders);
+
+        $row = $this->wrap('laravel_row');
+
+        return ', row_number() over ('.$over.') as '.$row;
+    }
+
     /**
      * Compile the "offset" portions of the query.
      *

src/Illuminate/Database/Query/Grammars/MySqlGrammar.php

@@ -4,6 +4,7 @@
 
 use Illuminate\Database\Query\Builder;
 use Illuminate\Support\Str;
+use PDO;
 
 class MySqlGrammar extends Grammar
 {
@@ -94,6 +95,82 @@ protected function compileIndexHint(Builder $query, $indexHint)
         };
     }
 
+    /**
+     * Compile a group limit clause.
+     *
+     * @param  \Illuminate\Database\Query\Builder  $query
+     * @return string
+     */
+    protected function compileGroupLimit(Builder $query)
+    {
+        return $this->useLegacyGroupLimit($query)
+            ? $this->compileLegacyGroupLimit($query)
+            : parent::compileGroupLimit($query);
+    }
+
+    /**
+     * Determine whether to use a legacy group limit clause for MySQL < 8.0.
+     *
+     * @param  \Illuminate\Database\Query\Builder  $query
+     * @return bool
+     */
+    public function useLegacyGroupLimit(Builder $query)
+    {
+        $version = $query->getConnection()->getReadPdo()->getAttribute(PDO::ATTR_SERVER_VERSION);
+
+        return ! $query->getConnection()->isMaria() && version_compare($version, '8.0.11') < 0;
+    }
+
+    /**
+     * Compile a group limit clause for MySQL < 8.0.
+     *
+     * Derived from https://softonsofa.com/tweaking-eloquent-relations-how-to-get-n-related-models-per-parent/.
+     *
+     * @param  \Illuminate\Database\Query\Builder  $query
+     * @return string
+     */
+    protected function compileLegacyGroupLimit(Builder $query)
+    {
+        $limit = (int) $query->groupLimit['value'];
+
+        $offset = $query->offset;
+
+        if (isset($offset)) {
+            $offset = (int) $offset;
+            $limit += $offset;
+
+            $query->offset = null;
+        }
+
+        $column = last(explode('.', $query->groupLimit['column']));
+
+        $column = $this->wrap($column);
+
+        $partition = ', @laravel_row := if(@laravel_group = '.$column.', @laravel_row + 1, 1) as `laravel_row`';
+
+        $partition .= ', @laravel_group := '.$column;
+
+        $orders = (array) $query->orders;
+
+        array_unshift($orders, ['column' => $query->groupLimit['column'], 'direction' => 'asc']);
+
+        $query->orders = $orders;
+
+        $components = $this->compileComponents($query);
+
+        $sql = $this->concatenate($components);
+
+        $from = '(select @laravel_row := 0, @laravel_group := 0) as `laravel_vars`, ('.$sql.') as `laravel_table`';
+
+        $sql = 'select `laravel_table`.*'.$partition.' from '.$from.' having `laravel_row` <= '.$limit;
+
+        if (isset($offset)) {
+            $sql .= ' and `laravel_row` > '.$offset;
+        }
+
+        return $sql.' order by `laravel_row`';
+    }
+
     /**
      * Compile an insert ignore statement into SQL.
      *