Compatibility of schemas with nested types (#504)

Hi,

Here is a minimal example of the issue I've encountered.

```julia
 struct A
    x::Int
end

struct B
    a::A
end

v = [B(A(i)) for i =1:3]

io = IOBuffer()
Arrow.write(io, v; file=false)
seekstart(io)
Arrow.append(io, v) # throws
```

I don't know if this is really necessary, or if I'm not using this
library properly, but this issue makes it difficult to append to arrow
files with nested types.

Since I've only added more cases where the call to `append` can succeed,
I do not think that this creates retro-compatibility issues.

Thanks for the review!

---------

Co-authored-by: Ben Baumgold <[email protected]>

Project.toml

@@ -17,7 +17,7 @@
 name = "Arrow"
 uuid = "69666777-d1a9-59fb-9406-91d4454c9d45"
 authors = ["quinnj <[email protected]>"]
-version = "2.7.1"
+version = "2.7.2"
 
 [deps]
 ArrowTypes = "31f734f8-188a-4ce0-8406-c8a06bd891cd"

src/append.jl

@@ -282,9 +282,31 @@ function is_equivalent_schema(sch1::Tables.Schema, sch2::Tables.Schema)
     for (t1, t2) in zip(sch1.types, sch2.types)
         tt1 = Base.nonmissingtype(t1)
         tt2 = Base.nonmissingtype(t2)
-        if t1 == t2 ||
-           (tt1 <: AbstractVector && tt2 <: AbstractVector && eltype(tt1) == eltype(tt2))
+        if t1 == t2
             continue
+        elseif tt1 <: AbstractVector && tt2 <: AbstractVector && eltype(tt1) == eltype(tt2)
+            continue
+        elseif isstructtype(tt1) && isstructtype(tt2)
+            is_equivalent_type_by_field(tt1, tt2)
+        else
+            return false
+        end
+    end
+    true
+end
+
+function is_equivalent_type_by_field(T1, T2)
+    n1 = fieldcount(T1)
+    n2 = fieldcount(T2)
+    n1 != n2 && return false
+
+    for i = 1:n1
+        fieldname(T1, i) == fieldname(T2, i) || return false
+
+        if fieldtype(T1, i) == fieldtype(T2, i)
+            continue
+        elseif isstructtype(T1) && isstructtype(T2)
+            is_equivalent_type_by_field(T1, T2) || continue
         else
             return false
         end

test/runtests.jl

@@ -1042,5 +1042,24 @@ end
                 @test tbl.f[2] === Foo493(4, 5)
             end
         end
+
+        @testset "# 504" begin
+            struct Foo504
+                x::Int
+            end
+
+            struct Bar504
+                a::Foo504
+            end
+
+            v = [Bar504(Foo504(i)) for i = 1:3]
+            io = IOBuffer()
+            Arrow.write(io, v; file=false)
+            seekstart(io)
+            Arrow.append(io, v) # testing the compatility between the schema of the arrow Table, and the "schema" of v (using the fallback mechanism of Tables.jl)
+            seekstart(io)
+            t = Arrow.Table(io)
+            @test Arrow.Tables.rowcount(t) == 6
+        end
     end # @testset "misc"
 end