MongoDB Golang: FindOne() with nested Bson.M failed for 10% attempts, Bson.M+Bson.D or a single Bson.M always work, why? - mongodb

Question

Need some help to figure out why nested bson.M doesn’t work occasionally in FindOne().
For the following Golang structs stored in a MongoDb collection for type A:
type A struct {
Id primitive.ObjectID
Random1 string
Parents []B
Random2 int
}
type B struct {
Id primitive.ObjectID
Random3 string
Children []C
Random4 int
}
type C struct {
Random5 string
Name Name
Random6 int
}
type Name struct {
FirstName string
LastName string
}
The following filter for FindOne(), which uses two bson.M, worked in most situations but failed to find a match in about 10% runs
filter1 := bson.M{
"parents.0.chilren.0.name": bson.M{
"first_name": "Mike",
"last_name": "Anderson",
},
}
The following two filters alway work, where filter 2 uses bson.D inside bson.M, and filter 3 just uses one bson.M
filter2 := bson.M{
"parents.0.chilren.0.name": bson.D{
{Key: "first_name", Value: "Mike"},
{Key: "last_name", Value: "Anderson"},
},
}
filter3 := bson.M{
"parents.0.chilren.0.name.first_name": "Mike",
"parents.0.chilren.0.name.last_name": "Anderson",
}
I found a similar question in https://jira.mongodb.org/browse/GODRIVER-877 but still don’t understand the differences or root cause. Thanks for the help!

Answer 1

bson.M is a map, thus, the order of its elements are not guaranteed. Since you are comparing parents.0.chilren.0.name, which is a structure, the search criteria has to match exactly, with the order of fields in the criteria matching the order fields in the database. That's why when you use bson.M, it sometimes matches and sometimes not. When the search criteria matches the database field order, you get results, otherwise you don't.
That's the reason why the criteria with bson.D always matches, because it is based on a slice, so the order of fields are preserved.
The last search criteria compares individual fields, not a structure, so the order of fields are immeterial.