How can I join multiple slices of the same entity into one slice?
Or how do I push a new entity value into a slice of the entity?
The go-wiki has a collection of SliceTricks that you will find useful.
For example,
Append Slice
a = append(a, b...)
Insert Value
s = append(s, 0)
copy(s[i+1:], s[i:])
s[i] = x
Push Value
a = append(a, x)
References:
Go Programming Language Specification:
The append builtin does both of that for you. Use it like:
a := []int{1, 2}
a = append(a, 3)
b := []int{4, 5}
a = append(a, b...)
// a now is []int{1, 2, 3, 4, 5}
If you need more information on how to use slices, I recommend reading Slices: usage and internals.