Highlighting the Problem
While builing out my side project, I wanted to really leverage and learn more about Ecto. One of the things I was working with was managing a list using Changesets. The struct in question is a simple one called Interval which logically just represents a start and end datetime.
I was working on a feature that would allow multiple Intervals to be shifted by some fixed number of minutes. So given one interval that represents 8-830am and another for 830am - 9am, and 30 mintues. This would reults in 830am-9am and 9am-930am. Simple enough right.
What if I were to tell you that I the following code would not properly determine that the intervals changed?
changes =
intervals_to_change
|> Enum.reduce([], fn interval, interval_acc ->
new_start = DateTime.add(interval.start, minutes, :minute)
new_end = DateTime.add(interval.end, minutes, :minute)
[%Interval{interval | start: new_start, end: new_end} | interval_acc]
end)
Ecto.Changeset.put_embed(changeset, :intervals, changes)
Yet this results in no changes being shown in the changeset
#Ecto.Changeset<action: nil, changes: %{}, errors: [], data: #Alpen.Intervals.IntervalList<>, valid?: true>
The fix looks identical, feel free to play a game of spot the difference in the meantime.
changes =
intervals_to_change
|> Enum.reduce([], fn interval, interval_acc ->
new_start = DateTime.add(interval.start, minutes, :minute)
new_end = DateTime.add(interval.end, minutes, :minute)
[%{id: interval.id, start: new_start, end: new_end} | interval_acc]
end)
Ecto.Changeset.put_embed(changeset, :intervals, changes)
#Ecto.Changeset<
action: nil,
changes: %{
intervals: [
#Ecto.Changeset<
action: :update,
changes: %{
start: ~U[2024-05-24 08:30:00Z],
end: ~U[2024-05-24 09:00:00Z]
},
errors: [],
data: #Alpen.Intervals.Interval<>,
valid?: true
>,
#Ecto.Changeset<
action: :update,
changes: %{
start: ~U[2024-05-24 09:00:00Z],
end: ~U[2024-05-24 09:30:00Z]
},
errors: [],
data: #Alpen.Intervals.Interval<>,
valid?: true
>
]
},
errors: [],
data: #Alpen.Intervals.IntervalList<>,
valid?: true
>
Structs are not Change Tracked
Confused I took a look through the docs and found this section on put_assoc/3. Specifically look at the section were it talks about when the associated data is a struct.
Different to passing changesets, structs are not change tracked in any fashion. In other words, if you change a comment struct and give it to put_assoc/4, the updates in the struct won’t be persisted. You must use changesets instead.