diff --git a/core/src/Ribasim.jl b/core/src/Ribasim.jl
index cb94448cb..0827e4a87 100644
--- a/core/src/Ribasim.jl
+++ b/core/src/Ribasim.jl
@@ -49,7 +49,13 @@ using Legolas: Legolas, @schema, @version, validate, SchemaVersion, declared
 using Logging: current_logger, min_enabled_level, with_logger
 using LoggingExtras: EarlyFilteredLogger, LevelOverrideLogger
 using MetaGraphsNext:
-    MetaGraphsNext, MetaGraph, label_for, labels, outneighbor_labels, inneighbor_labels
+    MetaGraphsNext,
+    MetaGraph,
+    label_for,
+    code_for,
+    labels,
+    outneighbor_labels,
+    inneighbor_labels
 using OrdinaryDiffEq
 using PreallocationTools: DiffCache, FixedSizeDiffCache, get_tmp
 using SciMLBase
diff --git a/core/src/allocation.jl b/core/src/allocation.jl
index ef7c52b4c..e1ec98e2c 100644
--- a/core/src/allocation.jl
+++ b/core/src/allocation.jl
@@ -13,8 +13,8 @@ function allocation_graph_used_nodes!(p::Parameters, allocation_network_id::Int)
         node_type = graph[node_id].type
         if node_type in [:user, :basin]
             push!(used_nodes, node_id)
-
-        elseif length(inoutflow_ids(graph, node_id)) > 2
+        elseif count(x -> true, inoutflow_ids(graph, node_id)) > 2
+            # use count since the length of the iterator is unknown
             push!(used_nodes, node_id)
         end
     end
diff --git a/core/src/solve.jl b/core/src/solve.jl
index 06c05a957..ff61b7a47 100644
--- a/core/src/solve.jl
+++ b/core/src/solve.jl
@@ -515,8 +515,8 @@ function valid_n_neighbors(node::AbstractParameterNode, graph::MetaGraph)::Bool
     for id in node.node_id
         for (bounds, edge_type) in
             zip((bounds_flow, bounds_control), (EdgeType.flow, EdgeType.control))
-            n_inneighbors = length(inneighbor_labels_type(graph, id, edge_type))
-            n_outneighbors = length(outneighbor_labels_type(graph, id, edge_type))
+            n_inneighbors = count(x -> true, inneighbor_labels_type(graph, id, edge_type))
+            n_outneighbors = count(x -> true, outneighbor_labels_type(graph, id, edge_type))
 
             if n_inneighbors < bounds.in_min
                 @error "Nodes of type $node_type must have at least $(bounds.in_min) $edge_type inneighbor(s) (got $n_inneighbors for node $id)."
diff --git a/core/src/utils.jl b/core/src/utils.jl
index 2174f7b7d..c124b4b26 100644
--- a/core/src/utils.jl
+++ b/core/src/utils.jl
@@ -74,6 +74,61 @@ function create_graph(db::DB)::MetaGraph
     return graph
 end
 
+abstract type AbstractNeighbors end
+
+"""
+Iterate over incoming neighbors of a given label in a MetaGraph, only for edges of edge_type
+"""
+struct InNeighbors{T} <: AbstractNeighbors
+    graph::T
+    label::NodeID
+    edge_type::EdgeType.T
+end
+
+"""
+Iterate over outgoing neighbors of a given label in a MetaGraph, only for edges of edge_type
+"""
+struct OutNeighbors{T} <: AbstractNeighbors
+    graph::T
+    label::NodeID
+    edge_type::EdgeType.T
+end
+
+Base.IteratorSize(::Type{<:AbstractNeighbors}) = Base.SizeUnknown()
+Base.eltype(::Type{<:AbstractNeighbors}) = NodeID
+
+function Base.iterate(iter::InNeighbors, state = 1)
+    (; graph, label, edge_type) = iter
+    code = code_for(graph, label)
+    local label_in
+    while true
+        x = iterate(inneighbors(graph, code), state)
+        x === nothing && return nothing
+        code_in, state = x
+        label_in = label_for(graph, code_in)
+        if graph[label_in, label].type == edge_type
+            break
+        end
+    end
+    return label_in, state
+end
+
+function Base.iterate(iter::OutNeighbors, state = 1)
+    (; graph, label, edge_type) = iter
+    code = code_for(graph, label)
+    local label_out
+    while true
+        x = iterate(outneighbors(graph, code), state)
+        x === nothing && return nothing
+        code_out, state = x
+        label_out = label_for(graph, code_out)
+        if graph[label, label_out].type == edge_type
+            break
+        end
+    end
+    return label_out, state
+end
+
 """
 Get the inneighbor node IDs of the given node ID (label)
 over the given edge type in the graph.
@@ -82,11 +137,8 @@ function inneighbor_labels_type(
     graph::MetaGraph,
     label::NodeID,
     edge_type::EdgeType.T,
-)::Vector{NodeID}
-    return [
-        label_in for label_in in inneighbor_labels(graph, label) if
-        graph[label_in, label].type == edge_type
-    ]
+)::InNeighbors
+    return InNeighbors(graph, label, edge_type)
 end
 
 """
@@ -97,11 +149,8 @@ function outneighbor_labels_type(
     graph::MetaGraph,
     label::NodeID,
     edge_type::EdgeType.T,
-)::Vector{NodeID}
-    return [
-        label_out for label_out in outneighbor_labels(graph, label) if
-        graph[label, label_out].type == edge_type
-    ]
+)::OutNeighbors
+    return OutNeighbors(graph, label, edge_type)
 end
 
 """
@@ -112,31 +161,31 @@ function all_neighbor_labels_type(
     graph::MetaGraph,
     label::NodeID,
     edge_type::EdgeType.T,
-)::Vector{NodeID}
-    return [
-        outneighbor_labels_type(graph, label, edge_type)...,
-        inneighbor_labels_type(graph, label, edge_type)...,
-    ]
+)::Iterators.Flatten
+    return Iterators.flatten((
+        outneighbor_labels_type(graph, label, edge_type),
+        inneighbor_labels_type(graph, label, edge_type),
+    ))
 end
 
 """
 Get the outneighbors over flow edges.
 """
-function outflow_ids(graph::MetaGraph, id::NodeID)::Vector{NodeID}
+function outflow_ids(graph::MetaGraph, id::NodeID)::OutNeighbors
     return outneighbor_labels_type(graph, id, EdgeType.flow)
 end
 
 """
 Get the inneighbors over flow edges.
 """
-function inflow_ids(graph::MetaGraph, id::NodeID)::Vector{NodeID}
+function inflow_ids(graph::MetaGraph, id::NodeID)::InNeighbors
     return inneighbor_labels_type(graph, id, EdgeType.flow)
 end
 
 """
 Get the in- and outneighbors over flow edges.
 """
-function inoutflow_ids(graph::MetaGraph, id::NodeID)::Vector{NodeID}
+function inoutflow_ids(graph::MetaGraph, id::NodeID)::Iterators.Flatten
     return all_neighbor_labels_type(graph, id, EdgeType.flow)
 end