Ensure that the order of migrations is reliable

[beppemarazzi]

AFAIK the order on wich migrations are executed is important and it's derived from lexicographical order of migration id. Since each migration id has a fixed length prefix with the timestamp, then the migrations are supposed to execute in that order.

But digging into the code, i've the suspect that there is no real guaranty about this order: in fact it appears to me that the migrations order relies on the order of insertion in a Dictionary.

https://github.com/aspnet/EntityFrameworkCore/blob/4f65388bf68312e16fe07e62f79d8fad4e242f2a/src/EFCore.Relational/Migrations/Internal/MigrationsAssembly.cs#L49-L73

but FCL documentation states:

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<TKey,TValue> structure representing a value and its key. The order in which the items are returned is undefined.

Isn't better to replace
var result = new Dictionary<string, TypeInfo>();
with
var result = new SortedList<string, TypeInfo>();

IMHO the same issue arises where MigrationsAssembly.Migrations is used:

https://github.com/aspnet/EntityFrameworkCore/blob/4f65388bf68312e16fe07e62f79d8fad4e242f2a/src/EFCore.Relational/Migrations/Internal/Migrator.cs#L180-L234

here you can add an explicit .OrderBy(m => m.Key) also when calculating migrationsToApply

Thanks

[ajcvickers]

@bricelam to investigate and consider for patch.

[inlineHamed]

Another thing is the system culture settings, If someone in a team use a calendar other than Georgian, migration orders get mixed. I have created an issue about it here (#12978)

[ajcvickers]

Triage; putting this into 2.2 since we should be more robust here.

[bricelam]

Conclusion: The order is reliable on currently supported runtime implementations, it's just documented as undefined. Updating with the proposed changes to be more robust/explicit.