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I have a question about how I should design my contract to achieve the following: I want to create products identifiers (can be struct or string) something like: "product#123abc". Then I want to map a struct to it (in my case this struct holds location information) Example:

struct Location {
    string city;
    string street;
    uint time;}

And finally I want to map multiple different location structs to my product so that I can call it like this:

function viewLocation(uint _productID, uint _location) public returns(struct){
    return Product[_productID].Location[_location].city;
}

Would it also be possible to just call the productID and show me all mapped locations to it? Something like this:

function viewLocation(uint _productID) public returns(array){
    return Product[_productID].Location;
}

If it doesn't work like I think about it, how would a workaround look like? I hope you can help me :)

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It is possible to do something like this:

struct Location {
    string city;
    string street;
    uint time;
}

struct Product {
    uint id;
    string name;
    mapping (uint => Location) locations;
}

mapping (uint => Product) products;

Then the implementations will be like:

function viewLocation(uint _productID, uint _location) public returns(struct memory Location){
    return products[_productID].locations[_location];
}

Unfortunately plain mappings cannot be iterated. But it is possible to combine in a more complex structure that can be iterated, see this for details and an implementation https://ethereum.stackexchange.com/a/13168.

Another alternative is to keep separated locations and products. And keep an array of location ids:

struct Product {
    uint id;
    string name;
    uint[] locations;
}

mapping (uint => Product) products;

mapping (uint => Location) locations;

The viewLocation will be more complex:

function viewLocation(uint _productID, uint _location) public returns(struct memory Location){
    // Check if _location is in return products[_productID]
    // Return locations[_location] if it is present
    // Fail otherwise
}
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