Split preloading-IN query into multiple queries (go-rel#283)

cursor.go

@@ -81,6 +81,7 @@ func scanMulti(cur Cursor, keyField string, keyType reflect.Type, cols map[inter
 	}
 
 	if !found && fields != nil {
+		// TODO: why a panic and not just an easily catchable error?
 		panic("rel: primary key row does not exists")
 	}
 

repository.go

@@ -1030,25 +1030,47 @@ func (r repository) preload(cw contextWrapper, records slice, field string, quer
 		path                                             = strings.Split(field, ".")
 		targets, table, keyField, keyType, ddata, loaded = r.mapPreloadTargets(records, path)
 		ids                                              = r.targetIDs(targets)
-		query                                            = Build(table, append(queriers, In(keyField, ids...))...)
+
+		inClauseLength = 999 // TODO: can this value come from the adapter, as it is dbms specific?
 	)
 
-	if len(targets) == 0 || loaded && !bool(query.ReloadQuery) {
-		return nil
-	}
+	// Create separate queries if the amount of ids is more than inClauseLength.
+	for {
+		if len(ids) == 0 {
+			break
+		}
 
-	var (
-		cur, err = cw.adapter.Query(cw.ctx, r.withDefaultScope(ddata, query, false))
-	)
+		// necessary check to avoid slicing beyond
+		// slice capacity
+		if len(ids) < inClauseLength {
+			inClauseLength = len(ids)
+		}
 
-	if err != nil {
-		return err
-	}
+		idsChunk := ids[0:inClauseLength]
+		ids = ids[inClauseLength:]
+
+		query := Build(table, append(queriers, In(keyField, idsChunk...))...)
+		if len(targets) == 0 || loaded && !bool(query.ReloadQuery) {
+			return nil
+		}
+
+		var (
+			cur, err = cw.adapter.Query(cw.ctx, r.withDefaultScope(ddata, query, false))
+		)
 
-	scanFinish := r.instrumenter.Observe(cw.ctx, "rel-scan-multi", "scanning all records to multiple targets")
-	defer scanFinish(nil)
+		if err != nil {
+			return err
+		}
 
-	return scanMulti(cur, keyField, keyType, targets)
+		scanFinish := r.instrumenter.Observe(cw.ctx, "rel-scan-multi", "scanning all records to multiple targets")
+		err = scanMulti(cur, keyField, keyType, targets)
+		scanFinish(nil)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 func (r repository) MustPreload(ctx context.Context, records interface{}, field string, queriers ...Querier) {