ruby: Add query for hoisting Rails ActiveRecord calls

This does not take assicoations into account.
It uses ActiveRecordModelFinderCall to identify relevant calls.
This class has therefor been made public.

ruby/ql/lib/codeql/ruby/frameworks/ActiveRecord.qll

@@ -254,9 +254,8 @@ private Expr getUltimateReceiver(MethodCall call) {
   )
 }
 
-// A call to `find`, `where`, etc. that may return active record model object(s)
-private class ActiveRecordModelFinderCall extends ActiveRecordModelInstantiation, DataFlow::CallNode
-{
+/** A call to `find`, `where`, etc. that may return active record model object(s) */
+class ActiveRecordModelFinderCall extends ActiveRecordModelInstantiation, DataFlow::CallNode {
   private ActiveRecordModelClass cls;
 
   ActiveRecordModelFinderCall() {

ruby/ql/src/queries/performance/CouldBeHoisted.qhelp

@@ -0,0 +1,22 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>
+When a Rails ActiveRecord query is executed in a loop, it is potentially an n+1 problem.
+This query identifies situations where an ActiveRecord query execution could be pulled out of a loop.
+</p>
+</overview>
+<recommendation>
+<p>If possible, pull the query out of the loop, thus replacing the many calls with a single one.
+</p>
+</recommendation>
+<example>
+<p>The following (suboptimal) example code queries the User object in each iteration of the loop:</p>
+<sample src="examples/straight_loop.rb" />
+<p>To improve the performance, we instead query the User object once outside the loop, gathereing all necessary information:</p>
+<sample src="examples/preload.rb" />
+</example>
+</qhelp>

ruby/ql/src/queries/performance/CouldBeHoisted.ql

@@ -0,0 +1,90 @@
+/**
+ * @name Could be hoisted
+ * @description Hoist Rails `ActiveRecord::Relation` query calls out of loops.
+ * @kind problem
+ * @problem.severity info
+ * @precision high
+ * @id rb/could-be-hoisted
+ * @tags performance
+ */
+
+// Possible Improvements;
+// - Consider also Associations.
+//   Associations are lazy-loading by default, so something like
+//   in a loop over `article` do
+//      `article.book`
+//   if you have 1000 articles it will do a 1000 calls to `book`.
+//   If you already did `article includes book`, there should be no problem.
+// - Consider instances of ActiveRecordInstanceMethodCall, for instance
+//   calls to `pluck`.
+import ruby
+private import codeql.ruby.AST
+import codeql.ruby.ast.internal.Constant
+import codeql.ruby.Concepts
+import codeql.ruby.frameworks.ActiveRecord
+private import codeql.ruby.TaintTracking
+
+string loopMethodName() {
+  result in [
+      "each", "reverse_each", "map", "map!", "foreach", "flat_map", "in_batches", "one?", "all?",
+      "collect", "collect!", "select", "select!", "reject", "reject!"
+    ]
+}
+
+class LoopingCall extends DataFlow::CallNode {
+  DataFlow::CallableNode loopBlock;
+
+  LoopingCall() {
+    this.getMethodName() = loopMethodName() and loopBlock = this.getBlock().asCallable()
+  }
+
+  DataFlow::CallableNode getLoopBlock() { result = loopBlock }
+}
+
+predicate happensInLoop(LoopingCall loop, DataFlow::CallNode e) {
+  loop.getLoopBlock().asCallableAstNode() = e.asExpr().getScope()
+}
+
+predicate happensInOuterLoop(LoopingCall outerLoop, DataFlow::CallNode e) {
+  exists(LoopingCall innerLoop |
+    happensInLoop(outerLoop, innerLoop) and
+    happensInLoop(innerLoop, e)
+  )
+}
+
+predicate happensInInnermostLoop(LoopingCall loop, DataFlow::CallNode e) {
+  happensInLoop(loop, e) and
+  not happensInOuterLoop(loop, e)
+}
+
+// The ActiveRecord instance is used to potentially control the loop
+predicate usedInLoopControlGuard(ActiveRecordInstance ar, DataFlow::Node guard) {
+  TaintTracking::localTaint(ar, guard) and
+  guard = guardForLoopControl(_, _)
+}
+
+// A guard for controlling the loop
+DataFlow::Node guardForLoopControl(ConditionalExpr cond, Stmt control) {
+  result.asExpr().getAstNode() = cond.getCondition().getAChild*() and
+  (
+    control.(MethodCall).getMethodName() = "raise"
+    or
+    control instanceof NextStmt
+  ) and
+  control = cond.getBranch(_).getAChild()
+}
+
+from LoopingCall loop, DataFlow::CallNode call
+where
+  // Disregard loops over constants
+  not isArrayConstant(loop.getReceiver().asExpr(), _) and
+  // Disregard tests
+  not call.getLocation().getFile().getAbsolutePath().matches("%test%") and
+  // Disregard cases where the looping is influenced by the query result
+  not usedInLoopControlGuard(call, _) and
+  // Only report the inner most loop
+  happensInInnermostLoop(loop, call) and
+  // Only report calls that are likely to be expensive
+  call instanceof ActiveRecordModelFinderCall and
+  not call.getMethodName() in ["new", "create"]
+select call, "This call happens inside $@, and could be hoisted.", loop, "this loop"

ruby/ql/src/queries/performance/examples/preload.rb

@@ -0,0 +1,14 @@
+# Preload User data
+user_data = User.where(login: repo_names_by_owner.keys).pluck(:login, :id, :type).to_h do |login, id, type|
+  [login, { id: id, type: type == "User" ? "USER" : "ORGANIZATION" }]
+end
+
+repo_names_by_owner.each do |owner_slug, repo_names|
+  owner_info = user_data[owner_slug]
+  owner_id = owner_info[:id]
+  owner_type = owner_info[:type]
+  rel_conditions = { owner_id: owner_id, name: repo_names }
+
+  nwo_rel = nwo_rel.or(RepositorySecurityCenterConfig.where(rel_conditions)) unless neg
+  nwo_rel = nwo_rel.and(RepositorySecurityCenterConfig.where.not(rel_conditions)) if neg
+end

ruby/ql/src/queries/performance/examples/straight_loop.rb

@@ -0,0 +1,8 @@
+repo_names_by_owner.map do |owner_slug, repo_names|
+    owner_id, owner_type = User.where(login: owner_slug).pluck(:id, :type).first
+    owner_type = owner_type == "User" ? "USER" : "ORGANIZATION"
+    rel_conditions = { owner_id: owner_id, name: repo_names }
+
+    nwo_rel = nwo_rel.or(RepositorySecurityCenterConfig.where(rel_conditions)) unless neg
+    nwo_rel = nwo_rel.and(RepositorySecurityCenterConfig.where.not(rel_conditions)) if neg
+  end